The Syrian operation includes American special forces operating in advisory roles on the ground. In Iraq, the US-led coalition has provided air support to government forces who are also assisted by Iranian-backed Shia militia.
While there is no indication that the two advances were deliberately timed to coincide, they show how a variety of enemies of Isis have been mobilising in recent months in what Washington and other world powers hope will be a decisive year of battle to destroy the group's pseudo-state.
The Syrian operation, which began on Tuesday after weeks of preparations, aims to drive Isis from the last stretch of the frontier with Turkey it controls.
"It's significant in that it's their last remaining funnel" to Europe, a US military official told Reuters. Isis has used the border for years to receive material and recruits from the outside world, and, more recently, to send militants back to Europe to carry out attacks.
An 80km stretch of terrain north of the town of Manbij is the only part of the Turkish frontier still accessible to the militants after advances by Kurdish fighters and President Bashar al-Assad's Government elsewhere.
A small number of US special operations forces will support the push on the ground to capture the "Manbij pocket", acting as advisers some distance back from the front lines, US officials said, discussing the plans on condition of anonymity.
The operation will also count on air power from the US-led coalition, which pounded Islamic State positions near Manbij with 18 strikes on Tuesday.
The Syrian Observatory for Human Rights, a group that reports on the conflict there with a network of sources on the ground, said Isis had been pushed out of 16 villages near Manbij.
US-led air strikes in support of the ground operation had killed 15 civilians including three children near Manbij in the last 24 hours, the Observatory said.
The assault is being carried out by an alliance known as the Syria Democratic Forces (SDF), which is composed of a powerful Kurdish militia called the YPG, and Arab combatants that have allied themselves with it.
The group, set up last year, is the main ground force to receive US backing in Syria, where Washington opposes Assad's Government and has had difficulty finding capable allies on the ground in the past.
US officials stressed that most of the fighting near Manbij would be carried out by Arabs, an emphasis apparently aimed at Turkey, which considers the Kurdish YPG to be foes.
"After they take Manbij, the agreement is the YPG will not be staying ... So you'll have Syrian Arabs occupying traditional Syrian Arab land," the first US official said.
However, the Observatory described much of the fighting so far as carried out by Kurds.
The operation is taking place ahead of an eventual push by the US-backed Syrian forces toward Raqqa, Isis' de facto Syrian capital, which, alongside Iraq's northern city of Mosul, is one of two main objectives to bring down the caliphate.
US President Barack Obama has authorised about 300 US special operations forces to operate on the ground inside Syria to help co-ordinate with local forces.
In a reminder of the risks, one US service member was injured north of Raqqa over the weekend, the Pentagon said.
A five-year-long, multi-sided civil war in Syria, in which global powers back enemy sides, has made it impossible to coordinate a single campaign against Isis there.
The US-backed advance comes some weeks after Assad government troops, with Russian and Iranian support, recaptured the ancient city of Palmyra from Isis.
In Iraq, where Abadi's Shia-led Government enjoys military backing both from the US and Washington's regional adversary Iran, the decision to pause at the gates of Fallujah postpones for now what is expected to be one of the biggest battles ever fought against Isis.
"It would have been possible to end the battle quickly if protecting civilians wasn't among our priorities," Abadi told military commanders at the operations room near the frontline in footage broadcast on state television.
"Thank God, our units are at the outskirts of Fallujah and victory is within reach."
Abadi first announced plans to assault Fallujah 10 days ago.
But with 50,000 civilians still believed to be trapped inside the city, the United Nations has warned that militants are holding hundreds of families in the centre as human shields.
After heavy resistance from Isis, the troops have not moved over the past 48 hours, keeping positions in Fallujah's mainly rural southern suburb of Naimiya, according to a Reuters TV crew reporting from the area.
No food in aid convoy
A humanitarian aid convoy yesterday entered the rebel-held Syrian town of Daraya, the Red Cross said, in the first such delivery since a regime siege began in 2012.
But the opposition said only medical supplies were in the delivery and British charity Save the Children said it was "shocking and completely unacceptable" that it excluded desperately needed food.
Last month the United Nations warned that if it did not see improvement on aid access to besieged areas by June 1, it would task the World Food Programme to carry out air drops of assistance in Syria.
The International Committee of the Red Cross said both United Nations and Syrian Arab Red Crescent staff were involved in yesterday's delivery.
Daraya was one of the first towns in Syria to erupt in demonstrations against the Government in 2012, and one of the first to be placed under a strict regime siege in late 2012.
An estimated 8000 people live in the town, which lies just a 15-minutes drive southwest of Damascus.
Despite intensifying appeals from its residents, the UN and rights groups, Syria's Government had so far repeatedly refused to allow aid into the town.
According to the UN, a total of 592,000 people live under siege in Syria - the majority besieged by regime forces - and another 4 million live in hard-to-reach areas.
More than 280,000 people have been killed since Syria's conflict erupted in March 2011.
- Reuters, AFP
