A close ally of Donald Trump said on Sunday the president was “thinking long and hard” about his plan to withdraw US troops from Syria and his commitment to defeating the Islamic State, and had provided reassurance in a White House meeting.

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Lindsey Graham did not say Trump had pledged to reconsider the withdrawal, which the senator earlier said he hoped to convince the president to do.

A retired commander of US and Nato troops in Afghanistan, meanwhile, criticised reported plans to withdraw thousands of Americans from that country and said the president himself was “immoral”.

Graham, from South Carolina, is an influential member of the Senate armed services committee who has opposed Trump on some foreign policy decisions while working to get close to him in a manner far removed from the early stages of the 2016 election, when the two men exchanged fierce insults.

Earlier this month, Graham used emotive language when he warned that the Syria withdrawal and mooted Afghanistan drawdown could pave “the way toward a second 9/11”. On Sunday morning, announcing his meeting with Trump, he was more measured, saying he would ask the president “to sit down with his generals and reconsider how to do this”.

At the same time, Stanley McChrystal, who commanded US and Nato forces in Afghanistan for about a year, said withdrawing up to half the 14,000 American troops still serving in that country would reduce the incentive for the Taliban to talk peace after more than 17 years of war. Talk of a pullout, he said, meant the US had “basically traded away the biggest leverage point we have”. Trump had also “rocked” America’s Afghan allies, he said.

Graham warned that removing all US forces from Syria would hurt national security by allowing Isis to rebuild, betraying US-backed Kurdish fighters and enhancing Iran’s ability to threaten Israel.

“Slow this down,” he said, speaking to CNN’s State of the Union. “Make sure we get it right. Make sure Isis never comes back. Don’t turn Syria over to the Iranians. I want to fight the war in the enemy’s backyard, not ours.”

Then he went to the White House. Whatever Trump told Graham, it seemed to have soothed him a little. Leaving the executive mansion, the senator told reporters: “We talked about Syria. He told me some things I didn’t know that made me feel a lot better about where we’re headed in Syria.

“We still have some differences but I will tell you that the president is thinking long and hard about Syria – how to withdraw our forces but at the same time achieve our national security interests.”

The Department of Defense says it is considering plans for a “deliberate and controlled withdrawal”. One option, according to a person familiar with the discussions, is for a 120-day pull-out period.

Graham has joined other Republicans and Democrats in criticizing Trump’s order for the pullout of all 2,000 US troops deployed to support mostly Kurdish anti-Isis fighters. Turkey views the Kurdish militia, known as the YPG, as a branch of its own Kurdish separatist movement and is threatening to launch an offensive against it.

US commanders are recommending YPG fighters be allowed to keep US-supplied weapons, according to US officials. That proposal would likely anger Turkey, where the national security adviser, John Bolton, will hold talks this week.

Trump decided on the Syria withdrawal in a phone call with Turkish president Recep Tayyip Erdoğan, ignoring aides, lawmakers and international allies. The decision prompted defense secretary Jim Mattis to resign, submitting a letter to Trump that also referenced reports of his Afghanistan plan and criticised his treatment of America’s friends.

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Like Mattis, McChrystal is a retired general. Discussing Afghanistan on ABC’s This Week on Sunday, he said: “If you tell the Taliban that we are absolutely leaving on date certain, cutting down, weakening ourselves, their incentives to try to cut a deal drop dramatically.”

He was also worried, he said, that the Afghan people would lose confidence in the US.

“I think we probably rocked them,” he said.

Asked what he would say if he were ever asked to join the Trump administration, McChrystal said: “I think it’s important for me to work for people who I think are basically honest, who tell the truth as best they know it.”

Asked if he thought Trump was immoral, he said: “I think he is.”

McChrystal has criticized a sitting president before. Barack Obama accepted his resignation in June 2010, after he made scathing remarks in a magazine article about officials including Obama and Vice-President Joe Biden.