US President Donald Trump has boasted of his "great and unmatched wisdom" and threatened to "destroy" Turkey's economy after appearing to green-light a Turkish invasion of northern Syria and abandoning the US-allied Kurds who helped fight against the Islamic State (IS) group.

Key points: The White House announced the withdrawal of troops from northern Syria on Sunday

The White House announced the withdrawal of troops from northern Syria on Sunday Republicans have criticised the move, which has prompted fears of a mass breakout of captured IS fighters

Republicans have criticised the move, which has prompted fears of a mass breakout of captured IS fighters Mr Trump later warned Turkey not to do anything he considered "off limits"

The White House on Sunday announced the withdrawal of troops from northern Syria, saying it was too costly to keep supporting the Kurdish People's Protection Units (YPG) militia, which had been an important American ally in the fight against IS jihadists.

Turkey's President Recep Tayyip Erdogan has long threatened that an operation against the Kurdish militants in northern Syria, who Ankara regards as terrorists, could come "any night without warning".

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Mr Trump's decision to effectively abandon the US's Kurdish allies has been roundly criticised by some members of his own party.

"A catastrophic mistake," House Republican Conference chair Liz Cheney said, while senator Lindsey Graham called it a "shot in the arm to the bad guys".

Ben Sasse, a Republican member of the Senate Intelligence Committee, urged Mr Trump to reconsider his decision to withdraw.

"If the President sticks with this retreat, he needs to know that this bad decision will likely result in the slaughter of allies who fought with us, including women and children," he said.

"And before Turkey butchers innocent Kurds, [Mr] Erdogan should carefully consider his privileged status as a NATO member."

In response, Mr Trump said the US had been a "sucker" to remain in the conflict.

"The Kurds fought with us, but were paid massive amounts of money and equipment to do so," he said on Twitter.

"It is time for us to get out of these ridiculous endless wars, many of them tribal, and bring our soldiers home.

"As I have stated strongly before, and just to reiterate, if Turkey does anything that I, in my great and unmatched wisdom, consider to be off limits, I will totally destroy and obliterate the Economy of Turkey (I've done before!)," he said.

Mr Trump said it was too costly for the US to continue its support of the YPG militia. ( Reuters: Goran Tomasevic )

Mr Trump was backed up by the Pentagon, which released its own statement opposing Turkish operations in Syria.

"The Department of Defense made clear to Turkey — as did the President — that we do not endorse a Turkish operation in northern Syria. The US Armed Forces will not support, or be involved in any such operation," Pentagon chief spokesman Jonathan Hoffman said in a statement.

Hoffman said Defense Secretary Mark Esper and the Joint Chiefs chairman, General Mark Milley, told their Turkish counterparts that unilateral military action in the Kurdish area of Syria "creates risks" for Turkey.

There are about 1,000 US troops in northern Syria.

The Kurdish Hawar news agency and the Britain-based Syrian Observatory for Human Rights said American troops were evacuating their positions near the towns of Ras al-Ayn and Tal Abyad on Monday.

Australia monitoring 'evolving' situation

The Prime Minister's Office has deferred questions on the matter to the defence and foreign affairs ministers' offices.

Defence Minister Linda Reynolds said Australia was in close contact with officials in the United States and other allies.

"The Australian Government is very aware of what's been happening and it saw the report on President Trump's comments," she said.

"It is too early yet to speculate on the consequences this might have for Australian policy in the region … as it is a very rapidly evolving, new situation.

"So we will continue to monitor and act accordingly when we have more information to hand."

Acting Opposition Leader Richard Marles echoed the Government's call for restraint from all parties and said Labor had requested a briefing on the situation.

"I really hope that all the players in what is a very complex situation are talking to each other with a view to reducing tensions rather than increasing them," he said.

"It is so important that as the days and weeks play out from here that we do not see the next chapter begin of Syria's misery."

Fears for refugee camp

Authorities in Kurdish-controlled Syria warn a planned Turkish invasion could lead to a mass breakout from the al-Hawl refugee camp, home to more than 60 Australian women and children.

The Kurdish-led Syrian Democratic Forces (SDF) say any Turkish military action will affect the security of the al-Hawl camp.

SDF spokesman Mustafa Bali told the ABC there was a real threat the prisoners could be freed to join sympathisers of the extremist group and recreate another IS caliphate.

Kamalle Dabboussy, the father of one of the women living in the camp, has urged the Australian Government to rescue the Australians as soon as possible.

"There hasn't actually yet been any action and they aren't that close so any action taking place may still allow the immediate area's safety for some time," he said.

"But as we speak it is still safe, governments can act and they should act immediately."

Mr Dabboussy wants the Government to bring Australians home from the Syrian refugee camp. ( ABC News: Dave Maguire )

Mr Dabboussy is leading a group of Australian families trying to bring their relatives home.

ABC's Four Corners accompanied him to Syria in August, when he visited with his daughter Mariam Dabboussy and her children for the first time in almost five years.

Earlier this month he spoke to the ABC about violence in the camp, which he said was unable to cope with the number people it housed.

Foreign Minister Marise Payne says while Australia remains concerned about their welfare, repatriation will only take place if it does not put government officials or forces at risk.

The EU warned that a military operation in north-east Syria would lead to the "massive displacement" of refugees and risked "severely undermining" political efforts aimed at ending Syria's eight-year-old conflict.

European governments are particularly worried that a Turkish offensive could allow some of around 2,000 foreign jihadists being held in Kurdish prisons to scatter and provide cover for Europeans among them who want to try to slip back home.

Mr Trump tweeted that it would be up to "Turkey, Europe, Syria, Iran, Iraq, Russia and the Kurds" to figure out what to do with the captured IS fighters.

Al-Hawl refugee camp is home to more than 60 Australian women and children.

ABC/Wires