The former defense secretary James Mattis has said Donald Trump’s abrupt withdrawal of US troops from the Syria-Turkey border has increased the chances of a resurgence of Islamic State. But the retired general passed up an opportunity to directly criticise the president.

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“If we don’t keep the pressure on,” Mattis told NBC’s Meet the Press, “then Isis will resurge. It’s absolutely a given that they will come back.”

After Mattis’s remarks were released, the Kurdish-led administration in northern Syria said 785 foreign individuals affiliated with Isis had escaped the camp where they were being held, following heavy Turkish shelling.

Trump announced the US withdrawal on Monday after a call with Turkish president Recep Tayyip Erdoğan. The surprise announcement prompted widespread accusations of a betrayal of Kurds allied to the US in war-torn Syria. Turkey, which regards some Kurdish groups as terrorists, swiftly attacked. The president also said Erdoğan would visit the White House.

Trump faced stringent attacks from both sides of the aisle. In Washington on Saturday night he held his ground, telling the conservative Values Voter Summit he was “an island of one”.

“We have to bring our great heroes, our great soldiers, we have to bring them home,” he insisted. “It’s time. It’s time.”

If we don’t keep the pressure on, then Isis will resurge. It’s absolutely a given that they will come back James Mattis

On Sunday morning, Trump warmed to his theme. The president said it was “very smart not to be involved in the intense fighting along the Turkish border, for a change”, amid a stream of tweets that included a startling statement: “Others may want to come in and fight for one side or the other. Let them!”

In more measured tones, defense secretary Mike Esper told CBS’ Face the Nation “it’s a very terrible situation over there” but insisted roughly 1,000 US troops would be evacuated in a “deliberate withdrawal”.

US forces are not yet out of harm’s way. The Washington Post reported that Turkish forces which shelled an area where US special forces troops remained on Friday had known for months they were there.

Brett McGurk, the former US envoy to the global coalition against Isis who resigned over Trump’s attempts to withdraw from Syria, told the Post: “Turkey wants us off the entire border region to a depth of 30km [20 miles]. Based on all the facts available, these were warning fires on a known location, not inadvertent rounds.”

Turkey is facing threats of US sanctions – reiterated by Trump in his speech on Saturday night – unless it calls off the incursion. Two of its Nato allies, Germany and France, have said they are halting weapons exports and the Arab League has denounced the operation.

But airstrikes and shelling continue in Kurdish areas and harrowing scenes among panicked and grieving refugees are being reported worldwide. More than 130,000 people have been displaced from rural areas around Tel Abyad and Ras al Ain as a result of the fighting, the United Nations said. Turkish forces and their Syrian allies seized large parts of the town of Suluk, the Syrian Observatory for Human Rights said on Sunday, the fifth day of the offensive.

On Saturday, CNN reported that earlier this week Gen Mazloum Kobani Abdi, head of the Kurdish-led Syrian Democratic Forces, told a senior US diplomat: “You have given up on us. You are leaving us to be slaughtered.”

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Also on Saturday, another SDF commander told a press conference: “The protection of Isis prisons will not remain our priority. The defence of our soil will be prioritised if [the] Turkish military continues its attacks.”

On Sunday, the Kurds said some Isis prisoners had escaped. In an apparent reference to Turkish-backed Syrian insurgents, the Kurds said mercenaries attacked a camp where Isis “elements” attacked guards and opened the gates.

“The brutal military assault led by Turkey and its mercenaries is now taking place near a camp in Ain Issa, where there are thousands from families of Isis,” the Kurds said, adding “some were able to escape after bombardments that targeted” the camp.

Mattis discussed the threat of an Isis resurgence on NBC’s Meet the Press with Chuck Todd, in an interview to be broadcast in full on Sunday.

“It’s in a situation of disarray right now,” he said in excerpts released by the broadcaster. “Obviously, the Kurds are adapting to the Turkish attacks. And we’ll have to see if they’re able to maintain the fight against Isis. It’s going to have an impact. The question is, how much?”

Asked if the US would regret Trump’s decision, Mattis said: “We have got to keep the pressure on Isis so they don’t recover.

“We may want a war over. We may even declare it over. You can pull your troops out as President Obama learned the hard way out of Iraq, but the ‘enemy gets the vote’, we say in the military. And in this case, if we don’t keep the pressure on, then Isis will resurge. It’s absolutely a given that they will come back.”

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Trump said this week any militant prisoners escaping from camps guarded by Kurds “will be escaping to Europe”. He also said the Kurds “didn’t help us in the second world war, they didn’t help us in Normandy, for example”.

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Mattis’s apparent disinclination to directly criticise the president, even as Syria spirals into ever worse chaos as a result of US actions, is in keeping with his approach since resigning in December 2018.

The retired US Marine Corps general has said he has a “duty of silence” regarding the president he served. That commitment has held despite Mattis having resigned, like McGurk, in response to an earlier attempt by Trump to pull US troops from Syria and in protest at his treatment of America’s allies.

In September, Mattis published a memoir, Call Sign Chaos. The book skirted his service to Trump, focusing instead on his career in the US armed forces.