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Media captionThe BBC's Martin Patience explains what's behind the conflict

Turkey is continuing its offensive against Kurdish-held areas in Syria.

Secretary Mark Esper warned of "serious consequences" while Treasury Secretary Steven Mnuchin held out the prospect of fresh sanctions.

President Donald Trump's decision to withdraw US troops effectively triggered the Turkish incursion.

He said he would like to negotiate a truce between Nato Turkey and the Kurds.

Turkish President Recep Tayyip Erdogan, however, says the military operation will continue.

According to the more than 100,000 people have fled their homes since the offensive began on Wednesday.

President Erdogan says he wants to create a "safe zone" in Syria that can be used by Syrian refugees.

As the crisis deepened, US troops said they had come under fire from Turkish artillery on Friday.

What is the latest?

The Kurdish-led Syrian Democratic Forces (SDF) – a key in the region – are facing numerous about 75 miles (120km) long, say correspondents.

Dozens of fighters from the SDF and pro-Turkish factions have been killed. Turkey's military confirmed the first death of a Turkish soldier and said three others had been wounded.

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Towns in northern Syria are targets for Turkish air strikes and artillery fire

At least 11 civilians have been killed and humanitarian groups.

On Friday, the Pentagon said its base near the northern Syrian town of Kobane had seen shell fire from Turkish positions.

Navy Captain Brook DeWalt said the area was "known by the Turks to have US forces present".

"All US troops are accounted for with no injuries," he said in a statement. "The US demands that Turkey avoid actions that could result in immediate defensive action."

Turkey denied deliberately targeting US forces.

What is the US doing?

Defense Secretary Esper insisted that the US had not abandoned its Kurdish allies and warned of unspecified "serious consequences" for Ankara if it did not halt the assault.

He accused President Recep Tayyip Erdogan of taking "impulsive action" and warned that the operation "risks the security of ISIS (Islamic State) prison camps".

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Media captionResidents who have fled the city of Ras al-Ain wait at a shelter

Treasury Secretary Steven Mnuchin said President Trump had authored officials to draft what he called "very significant" new economic sanctions against Turkey.

"We can shut down the Turkish economy if we need to," he said.

President Trump spoke briefly about the situation as he prepared for a rally on Friday, saying: "We do not want to kill a lot of people … if we have to use sanctions we will."

Meanwhile in Congress, lawmakers from both sides of the political divide

A bill to issue sanctions against Turkish officials and banks involved in the offensive was introduced by the Democratic chairman of the House of Representatives Foreign Affairs Committee, Eliot Engel, and the Republican Committee's ranking Mike McCaul.

What has Turkey said?

"President Erdogan said on Friday. "We will not step back."

He has been attacked by some $ 3.6 million of Syrian refugees it hosts to Europe.

What are the humanitarian fears?

A refugee crisis is developing. The UN's Office for the Co-ordination of Humanitarian Affairs (OCHA) gave the figure of 450,000 people could be forced to move.

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Many families are fleeing their homes as Turkish forces advance

According to the workers on the ground, the vast majority of civilians have fled Tal Abyad and those who are afraid of their lives.

OCHA said the Turkish bombardment. Thousands of people in the Hassakeh region, it reports.

Turkey wants to create a "safe zone" running for 480km (300 miles) along the Syrian side of the border but says it will be more than 32km limit.