The prospect of a US strike on Syria in retaliation for an alleged chemical attack, coupled with a missile raid apparently carried out by Israel that killed Iranian military personnel, has underscored the risk that the conflict is on the brink of a dangerous escalation.

Syria and Russia accused Israel of carrying out the strike against a Syrian base where Iranians were stationed. More than ever, Syria is in danger of becoming an arena for the settling of scores among world powers.

Speaking at a meeting with military leaders and national security advisers, US President Donald Trump said he would make a decision today "or very shortly thereafter" on a response, adding that the United States had "a lot of options militarily" on Syria.

"But we can't let atrocities like we all witnessed ... we can't let that happen in our world ... especially when we're able to because of the power of the United States, the power of our country, we're able to stop it."


The US envoy to the United Nations, Nikki Haley, said Washington "will respond" to the attack regardless of whether the UN Security Council acts or not.

She said: "The United States is determined to see that the monster who dropped chemical weapons on the Syrian people held to account."

Despite Trump's earlier warning that Syrian President Bashar al-Assad would pay a "big price" for his military's alleged use of poison, analysts questioned whether US strikes would influence the course of events on the battlefield and stem the seeming inevitability of a Syrian government victory over its opponents.

Today, Syrian rebels began evacuating the Damascus suburb where the alleged poison gas attack took place, after agreeing to a surrender deal that will restore government control over the area for the first time in six years.

US strikes are not going to alter the Assad Government's trajectory "and they may make things worse," said Emile Hokayem of the International Institute for Strategic Studies.

"There might be a narrow, self-satisfying strike, but as long as there is no bigger perspective or broader strategy for the whole conflict, it may just fuel escalation without meeting any objective." He added, "The time for intervention has passed."

Syria was already on edge, braced for military retaliation from the United States, when missiles struck an air base near Palmyra in the east of the province of Homs in the pre-dawn hours, prompting accusations from the Syrian Government that US forces were responsible. After the Pentagon issued a strong denial, Russia and Syria then said it was Israel that had attacked the T-4 base.

Iran's Fars News Agency said four Iranians were among at least 14 people reportedly killed at the base, which also houses Russians and members of the Lebanese Hizbollah militia. According to Russia's Defence Ministry, Israel carried out the attack by launching eight guided missiles from two F-15 planes, and Syria shot down five of the missiles.

Israel did not acknowledge carrying out the strike.

In Washington, Trump said his team was still debating whether to punish Damascus for the alleged use of chemical weapons in the Sunday attack on the town of Douma, the last major rebel-held urban stronghold in the suburb of Eastern Ghouta.

Videos of the incident posted online showed piles of crumpled bodies, many of them women and children, crammed together in an apartment building, wide-eyed and with foam on their mouths, suggesting a poisonous gas had killed them. The Syrian American Medical Society said it had counted 49 people killed in the attack, and the toll may rise as more bodies are identified.

Russia and Syria deny that chemical weapons were used.

The main rebel group in the area, Jaish al-Islam, had been holding out for a settlement that would allow it to remain and join a peace process proposed by the Russians under which rebel-held territories would eventually reconcile with the Assad government.

After the alleged chemical attack, the rebels relented, agreeing to evacuate to rebel-held areas in the north and allow the government to retake control of the enclave, residents said.

The attack came as the final straw following weeks of sustained airstrikes that killed hundreds of people and injured thousands.

The bombardment had kept more than 100,000 people huddled in basements and shelters, said a medical student in Douma who has worked with the opposition and spoke on the condition of anonymity for safety reasons.

"When you know there is no one to support you and when you know that the whole world is going to be silent no matter how many times you have been targeted, your choice will be to say: 'Okay, stop the killings and I will do whatever you like,' " the student said. "People can no longer handle it."

Today the rebels began boarding buses for northern Syria alongside several thousand civilians who fear being detained for their opposition activities once government control returns.

The departure was broadcast by state television and trumpeted as yet another major military victory for Assad over his opponents.

Eastern Ghouta was the last significant area controlled by the rebels in the vicinity of the capital, and though a government victory was a foregone conclusion after troops launched a major offensive in February, the capitulation came as yet another milestone in the Assad Government's march toward defeating its opponents.

At the same time, Trump's declaration last week that he wants to pull US troops out of northeastern Syria further undermines the impact that strikes might have on slowing the government's progress, said Faysal Itani of the Atlantic Council in Washington.

"A president who says he wants to get the hell out of Syria is not really in a position to threaten the military progress of the regime," he said.

"If Assad has boxed us into a position where we've got to throw some missiles at him, it doesn't really change the picture."

A small attack such as the one a year ago conducted in retaliation for a sarin gas attack that killed civilians in the northern Syria town of Khan Sheikhoun wouldn't make a difference, he said. A larger one would run the risk of confrontation with Iran and Russia, who have both repeatedly expressed their desire to see the United States leave Syria.

The small US force of about 2000 troops deployed in northeastern Syria alongside Syria's Kurds are particularly vulnerable to revenge attacks by both Iran and Syria, he said. Iran also could also push back against the United States in places such as Iraq, where US troops are present.

Russia has warned that US strikes in Syria would have "grave consequences," according to a Foreign Ministry statement. Given the heightened tensions between Russia and the United States on other issues, Russian President Vladimir Putin may seize on US strikes as an opportunity to leverage a confrontation and force the United States to the negotiating table, said Vladimir Frolov.

"The Cuban missile crisis might be the template Putin is looking at right now," he said. "Force a military showdown then call Trump to a summit to decrease tensions."

"Russia wants the US out of Syria as soon as possible, so if we have a clash and Trump retreats, Putin scores twice," he added.

Israel meanwhile has repeatedly expressed its own concerns about the expanding Iranian military presence in Syria as the Syrian Government consolidates its control, and may have seen Trump's threats as an opportunity to piggy back onto America's threats, said Michael Horowitz, a senior analyst at Le Beck International, a Middle East-based geopolitical and security consultancy.

"The timing of the strike isn't coincidental," he said.

"By striking [Syrian President Bashar al-Assad] and his Iranian allies just a day after Trump warned them of the price they would pay. . . Israel mitigates the risk of an Iranian response," he said.

"Israel has been trying to convince Washington to adopt a more pro-active, anti-Iran strategy in Syria, and certainly sees Trump's rhetoric in the wake of the chemical attack as an opportunity."

Russia and Syria have consistently denied all the allegations of chemical attacks during the seven year old Syrian war, and in this instance have accused the rebels of staging a "false flag" incident to trigger US intervention.

As Russian troops moved into the area today in the wake of the evacuating fighters, Syrian doctors visited the site of the alleged chemical attack and found no evidence that poisonous gases had been used, Russia's defence ministry said in a statement.

The Eastern Ghouta area has been under rebel control for the past six years and completely surrounded by the government for nearly five years, making it impossible to independently verify the accounts of a chemical attack.

- additional reporting AP, Reuters