The US has conducted air strikes on one of its own bases as part of the withdrawal of forces from northern Syria this week.

Two US air force F-15 jets struck the base, located at a former factory for French cement company Lafarge, near the Turkish border on Wednesday.

According to CNN, US Army Colonel Myles Caggins said Kurdish forces had vacated the compound the day before, setting the place on fire and leaving as Turkish-backed militias advanced into the area, reports News.com.au.

NBC News foreign affairs analyst Brett McGurk said the bombing of formerly held positions to prevent their seizure was a "break glass" procedure representing an "extreme worst-case scenario".

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Earlier this month, US Defense Secretary Mark Esper announced Preisdent Donald Trump had ordered the partial withdrawal of US troops from northern Syria, opening the door for neighbouring Turkey to begin attacking Kurdish-led forces occupying the region.

The Syrian Democratic Forces (SDF) — an alliance primarily led by Kurdish militias — had been important allies in the fight against ISIS, despite ties to the Kurdistan Workers' Party (PKK), which is considered a terrorist organisation by Turkey, the US, and 29 other nations.

President Donald Trump arrives at Andrews Air Force Base, Friday, Oct. 18. Photo / AP
President Donald Trump arrives at Andrews Air Force Base, Friday, Oct. 18. Photo / AP

The withdrawal process has now been expedited and expanded, with Secretary Esper announcing last Sunday that the US would withdraw all of its roughly 1,000 troops from the region.

"We have American forces likely caught between two opposing, advancing armies and it's a very untenable situation," Secretary Esper said.

"I spoke with the President last night, after discussions with the rest of the national security team, and he directed that we begin a deliberate withdrawal of forces from northern Syria."

Turkish President Recep Tayyip Erdogan said he wants to drive the SDF out of the border region and create a "safe zone" extending roughly 30 kilometres south of the border, in the hope that some of the 3.6 million Syrian refugees in Turkey can be resettled there.

Conflict is reportedly continuing in the area despite a US-brokered five-day ceasefire announced on Thursday.

There have been reports of shelling being heard near the border as well as ground skirmishes.

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The ceasefire appears to have been the result of the US threatening sanctions against Turkey.

"You don't want to be responsible for slaughtering thousands of people, and I don't want to be responsible for destroying the Turkish economy — and I will," President Trump wrote in a letter to President Erdogan encouraging him to negotiate with SDF commander Mazlum Kobanê last week.

"History will look upon you favourably if you get this done the right and humane way," President Trump added. "It will look upon you forever as the devil if good things don't happen. Don't be a tough guy. Don't be a fool!"

President Erdogan said the letter showed a lack of respect that Turkey wouldn't forget.

"When the time comes necessary steps will be taken," President Erdogan said, according to BBC North America editor Jon Sopel.

Already Turkey's incursion into Syria has attracted condemnations and concerns from human rights groups.

Amnesty International said yesterday that Turkey-backed Syrian armed groups had displayed a shameful disregard for civilian lives, carrying out serious violations and war crimes, including summary killings and unlawful attacks.

A U. S. Air Force F-15 fighter jet takes off from Incirlik Air Base near Adana, Turkey. Photo / AP
A U. S. Air Force F-15 fighter jet takes off from Incirlik Air Base near Adana, Turkey. Photo / AP

"Turkish military forces and their allies have displayed an utterly callous disregard for civilian lives, launching unlawful deadly attacks in residential areas that have killed and injured civilians," said Amnesty International secretary-general Kumi Naidoo.

Kurdish authorities said at least 218 civilians, including 18 children, had been killed in Syria since the offensive started.

But Amnesty International also noted Turkish claims that 18 civilians had been killed by Kurdish forces in Syria firing mortars over the border.

"If Kurdish forces are firing imprecise explosive weapons into civilian areas in Turkey this would violate international humanitarian law. They should stop such unlawful attacks immediately," Amnesty International said.

The United Nations' Organisation for the Prohibition of Chemical Weapons also told the Guardian yesterday it's looking into claims Turkish forces had used white phosphorous on civilians.

— With wires