Two days after imposing sanctions on Turkey over its military offensive in northern Syria, US President Donald Trump said that the Turkish operation is not Washington's problem.

In a meeting with Italian President Sergio Mattarella at the White House on Wednesday, Trump defended Washington's troop pullback in northern Syria that began last week, allowing Turkey to advance against Washington-allied Kurdish fighters in the area.

"We are not a policing agent. It is time for us to go home," Trump said. "If Turkey goes into Syria it is between Turkey and Syria. It’s not our problem."

Trump also said that the Syrian Kurds, who had been Washington's main allies in the fight against the Islamic State (IS) group, were "not angels".

'They're not angels... We paid a lot of money for them to fight with us, and that's OK' - Donald Trump on the Kurds

"They're not angels. You have to go back and take a look, but they fought with us," the US president said.

"We paid a lot of money for them to fight with us, and that's OK. They did well when they fought with us. They didn't do so well when they didn't fight with us."

Later on Wednesday during a news conference, Trump said that the Turkey-based Kurdistan Workers' Party (PKK) was a greater threat to the world 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," Trump said, using an alternative acronym for the IS group.

Turkey says the Syrian Kurdish YPG, a major faction in the US-backed Syrian Democratic Forces (SDF), is an offshoot of the PKK.

Trump has defended what dozens of Congress members have called a betrayal of Washington's Kurdish allies, by insisting that the US needs to withdraw itself from endless wars.

"Our soldiers are not in harm's way, as they shouldn't be, as two countries fight for land that has nothing to do with us," Trump said on Wednesday.

On Thursday, US Vice President Mike Pence is set to meet with Turkish President Recep Tayyip Erdogan in Turkey.

Erdogan says Turkey won't declare ceasefire in northern Syria until Kurdish forces leave Read More »

The White House had said that Pence plans to urge Turkey to call an immediate ceasefire in Syria and to work towards a negotiated settlement with the Kurdish-led SDF.

Robert O'Brien, the White House national security adviser, arrived in Turkey on Wednesday, aiming to meet Foreign Minister Mevlut Cavusoglu.

As the Trump administration tries to calm the situation in Syria through talks with Ankara, a leaked letter revealed on Wednesday that the US president had advised his Turkish counterpart last week not to be a "tough guy", a "fool" or a "devil".

The letter dated 9 October was sent to Turkey's Erdogan in an attempt to persuade Ankara into a deal with Kurkish forces.

"History will look upon you favorably if you get this done the right and humane way. It will look upon you forever as the devil if good things don't happen," Trump told Erdogan in the letter, which was first reported by the Fox Business Network.

Speaking to reporters in the Turkish parliament earlier on Wednesday, Erdogan said he was reevaluating his planned visit to Washington in November, but may visit Russia, Reuters news agency reported.

2020 candidates slam Trump's decision

At an election debate late on Tuesday, the leading Democratic presidential candidates slammed Trump's decision to move American troops from northern Syria.

"It’s been the most shameful thing any president has done in modern history in terms of foreign policy," former vice president Joe Biden said.

Senator Bernie Sanders said Turkey, which is a member of NATO, is no longer a US ally. He also slammed the US pullout in northern Syria, saying that Trump is compromising America's standing in the world by abandoning the Kurds.

"Now you tell me what country in the world will trust the word of the president of the United States?" the senator said.

For her part, Senator Amy Klobuchar invoked Israel to criticise Trump's handling of the situation in Syria.

"Think about our other allies, Israel. How do they feel right now? Donald Trump is not true to his word when they are a beacon of democracy in the Mideast."