President Donald Trump on Wednesday defended his decision to pull US troops out of Syria, calling it "strategically brilliant," and denied giving Turkey a "green light" to launch operations against Kurdish militants.

"President (Recep Tayyip) Erdogan's decision didn't surprise me because he's wanted to do that for a long time," Trump told reporters at the White House. "He's been building up troops on the border with Syria for a long time."

The US president also disparaged the Kurdish allies he abandoned in the face of the Turkish offensive, saying they were "not angels," as well as Republican critics at home of his abrupt withdrawal.

Trump has come under intense fire for effectively giving Turkey a green light to invade northern Syria, but he dismissed those concerns.

"I didn't give him a green light," Trump said of a telephone conversation he held with Erdogan prior to the Turkish incursion. "Just the opposite of a green light.

"I wrote a letter right after that conversation, a very powerful letter," he said.

Trump spoke as Vice President Mike Pence and Secretary of State Mike Pompeo prepared to travel to Turkey to try to persuade Erdogan to halt the offensive.

Trump emphasized that US troops were out of harm's way and that the fallout from the US withdrawal was for Syria, Turkey and Russia to work out among themselves.

"I view the situation on the Turkish border with Syria to be, for the United States, strategically brilliant," Trump said, alongside visiting Italian President Sergio Mattarelli.

"Our soldiers are out of there, our soldiers are totally safe. They've got to work it out," he said.

"Turkey has gone into Syria. If Turkey goes into Syria, that's between Turkey and Syria - it's not between Turkey and the United States, like a lot of stupid people would like you to believe."

PKK 'worse' than Islamic State

Trump expressed confidence that US nuclear weapons stored at Turkey's Incirlik air base were secure, despite flaring tensions between Ankara and its NATO allies over the Syrian incursion.

And he minimized the escape of Islamic State fighters who had been held by the Kurds as an attempt to "make us look like, 'Oh, gee, we have to get right back in there.'"

Critics have warned their escape gives the jihadists an opportunity to regroup after having lost their caliphate in Syria to US forces and their Kurdish allies, and the UN Security Council on Wednesday adopted a unanimous statement warning of the risk of a jihadist "dispersion" in the region.

"You have a lot of countries over there that have power and that hate ISIS very much, as much as we do. So I think we're in a very strategically good position," Trump said.

"And the Kurds are much safer right now but the Kurds know how to fight and as I said, they aren't angels. They are not angels if you take a look," he said.

Trump said the Kurdish rebel PKK, who have waged a decades-long insurgency against Ankara, were "probably" a bigger terror threat than the Islamic State group.

"The PKK, which is a part of the Kurds, as you know, is probably worse at terror and more of a terrorist threat in many ways than ISIS," he said.

Tens of thousands have died since the Kurdish Workers Party (PKK) took up arms against the Turkish state in 1984. The PKK is considered a terror group by Ankara, the United States and the European Union.

Trump lashed out at critics of his withdrawal decision, particularly Senator Lindsey Graham, an influential Republican from South Carolina who has had the president's ear in recent months.

"Lindsey Graham would like to stay in the Middle East for the next thousand years with thousands of soldiers fighting other people's wars," Trump said. "I want to get out of the Middle East."

"The people of South Carolina don't want us to get into a war with Turkey, a NATO member, or with Syria," he said. "Let them fight their own wars. They've been fighting for a thousand years." Graham shot right back, in comments to US reporters, that Trump was making "the biggest mistake of his presidency" in Syria. "I will not ever be quiet about matters of national security," he said.

Copyright Agence France-Presse, 2019