France, Germany and the UK have called for a meeting of the United Nations Security Council amid broad international criticism of Turkey's military assault on Kurdish forces in north eastern Syria.

French President Emmanuel Macron has urged Turkey to immediately end its attack, saying it risked boosting Islamic State extremists.

Turkey pounded the area with jets and artillery and sent in troops three days after US President Donald Trump told his Turkish counterpart Recep Tayyip Erdogan that he would withdraw US forces effectively serving as a buffer.

At least 15 people are reported to have died.

Turkish troop and allied Syrian rebels have now encircled two border towns in north-east Syria held by US-backed Kurdish forces, a spokesperson for the rebels has said.

The Turkish assault on the Syrian Kurdish forces, with which the US partnered to combat the Islamic State group, has sparked fears that it could lead to captured fighters they held escaping and reconstituting the group.

President Trump's decision to withdraw US troops triggered furious criticism from politicians in Washington hoping to protect the Kurdish forces.

A bipartisan group of senators introduced legislation that would freeze all US assets of Turkey's leadership, including President Erdogan, and impose sanctions on entities that do business with Turkey's military until Ankara withdraws from Syria.

Lindsey Graham, a senior Republican senator who is usually a steadfast supporter of Mr Trump, had warned Turkey of "sanctions from hell" if it went ahead with the invasion.

"Pray for our Kurdish allies who have been shamelessly abandoned by the Trump Administration. This move ensures the reemergence of ISIS," Mr Graham wrote on Twitter, using an acronym for the Islamic State group.

Defending his decision, President Trump said the Kurds did not "help us in Normandy" during World War II.

He threatened to "wipe out" Turkey's economy if the assault was not carried out "in as humane a way as possible".

"If (Erdogan) does it unfairly, he is going to pay a big economic price," Mr Trump told reporters, adding that he'd "do far more than sanctions".

But he distanced himself from Mr Graham, saying the senator "would like us to stay there for the next 200 years".

Democrats also roundly criticised President Trump as abandoning allies, even though some agreed that US troops should leave.

"These sanctions will have immediate, far-reaching consequences for Erdogan and his military," said Senator Chris Van Hollen, a Democrat who teamed up with Mr Graham.

President Trump signalled tentative support for sanctions but could veto the legislation if it is approved, setting up a showdown with Congress.

NATO chief Jens Stoltenberg urged Turkey to show "restraint" in its operation against Kurdish forces in Syria, warning that the fight against the Islamic State group should not be put at risk.

The SDF called on the international community to impose a no-fly zone to protect against "an imminent humanitarian crisis".

An official affiliated with Kurdish-led fighters said it would prevent civilian casualties.

US takes custody of two 'high-value' jihadists from Syria Kurds

Two "high-value" jihadists held by Syrian Kurds have been taken into US custody and moved out of the country, a defence official has said, as reports identified them as notorious British fighters dubbed "The Beatles".

"I can confirm that we've taken custody of two high-value ISIS individuals from the SDF," the official said on condition of anonymity, referring to the Islamic State group and the Kurdish-led Syrian Democratic Forces that held the captured jihadists.

"They have been moved out of Syria and are in a secure location," the official said, without identifying where.

"They are being held in military custody pursuant to the law of war."

US media reports identified the two as part of an extremely violent all-British four-man cell that kidnapped and tortured foreigners, including journalists, at the height of Islamic State group's power in Syria and Iraq.

One other was killed in a drone strike and the fourth is imprisoned on terror charges in Turkey.