Turkey stepped up its air and artillery strikes on Kurdish militia in northeast Syria on Friday, escalating an offensive that has drawn warnings of humanitarian catastrophe and turned Republican lawmakers against U.S. President Donald Trump.
The incursion, launched after Trump withdrew U.S. troops who had been fighting alongside Kurdish forces against Islamic State militants, has opened a new front in the eight-year-old Syrian civil war and drawn fierce international criticism.
In Washington, Trump - fending off accusations that he abandoned the Kurds, loyal allies of the United States - suggested that Washington could mediate in the conflict, while also raising the possibility of imposing sanctions on Turkey.
On Friday, Turkish warplanes and artillery struck around Syria's Ras al Ain, one of two border towns that have been the focus of the offensive. Reuters journalists heard gunfire there from across the frontier in the Turkish town of Ceylanpinar.
A convoy of 20 armoured vehicles carrying Turkish-allied Syrian rebels entered Syria from Ceylanpinar. Some made victory signs, shouting "Allahu akbar" (God is greatest) and waving Syrian rebel flags as they advanced towards Ras al Ain.
Some 120 km (75 miles) to the west, Turkish howitzers resumed shelling near the Syrian town of Tel Abyad, a witness said.
"In these moments, Tel Abyad is seeing the most intense battles in three days," Marvan Qamishlo, a spokesman for the Kurdish-led Syrian Democratic Forces (SDF) said.
Overnight, clashes erupted at different points along the border from Ain Diwar at the Iraqi frontier to Kobani, more than 400 km to the west. Turkish and SDF forces exchanged shelling in Qamishli among other places, the SDF's Qamishlo said.
"The whole border was on fire," he said.
Turkish forces have seized nine villages near Ras al Ain and Tel Abyad, said Rami Abdulrahman, director of the Syrian Observatory for Human Rights, which monitors the war.
At least 32 fighters with the SDF and 34 Turkey-backed Syrian rebels have been killed in fighting, while 10 civilians have been killed, Abdulrahman said. The SDF said 22 of its fighters were killed on Wednesday and Thursday.
Turkey says it has killed hundreds of SDF fighters in the operation and one Turkish soldier has been killed.
In Syria's al Bab, some 150 km west of the offensive, some 500 Turkish-backed Syrian fighters were set to head to Turkey to join the operation, CNN Turk reported. It broadcast video of them performing Muslim prayers in military fatigues, their rifles laid down in front of them, before departing for Turkey.
Turkey says the purpose of its assault is to defeat the Kurdish YPG militia, which it sees as an enemy for its links to insurgents in Turkey. It says it aims to set up a "safe zone" inside Syria, where it can resettle many of the 3.6 million refugees it has been hosting.
Turkey's President Tayyip Erdogan criticised Europe for failing to support the Turkish offensive and threatened to send refugees to Europe if the EU did not back him.
European Council President Donald Tusk responded on Friday by chastising Erdogan for making the threat.
"Turkey must understand that our main concern is that their actions may lead to another humanitarian catastrophe," he said.
The International Rescue Committee aid group says 64,000 people in Syria have fled in the first days of the campaign.
The Kurdish YPG is the main fighting element of the Syrian Democratic Forces (SDF) which have acted as the principal allies of the United States in a campaign that recaptured territory held by the Islamic State group.
The SDF now holds most of the territory that once made up Islamic State's "caliphate" in Syria, and has been keeping thousands of Islamic State fighters in jail and tens of thousands of their family members in camps.
A camp sheltering more than 7,000 displaced people in northern Syria is to be evacuated and there are talks on moving a second camp for 13,000 people including Islamic State fighters' familes, after both were shelled, Kurdish-led authorities said.
Medecins Sans Frontieres said a hospital in Tel Abyad had been forced to shut after most of its staff fled from bombings over the past 24 hours.
RARE REPUBLICAN CRITICISM OF TRUMP
In the United States, Trump's decision to withhold protection from the Kurds has been one of the few issues to prompt criticism from his fellow Republicans, including leading allies on Capitol Hill such as Senator Lindsey Graham.
Trump said in a Twitter post on Thursday: "We have one of three choices: Send in thousands of troops and win Militarily, hit Turkey very hard Financially and with Sanctions, or mediate a deal between Turkey and the Kurds!".
"I hope we can mediate," Trump said when asked about the options by reporters at the White House.
Without elaborating, he said the United States was "going to possibly do something very, very tough with respect to sanctions and other financial things" against Turkey.
Western countries' rejection of the Turkish offensive creates a rift within the NATO alliance, in which Turkey is the main Muslim member.
NATO Secretary General Jens Stoltenberg said after talks with Foreign Minister Mevlut Cavusoglu in Istanbul that he expected Turkey to act with restraint in Syria. Cavusoglu said Ankara expected "strong solidarity" from the alliance.
Stoltenberg also told reporters the international community must find a sustainable solution for Islamic State prisoners in Syria.
French Foreign Minister Jean-Yves Le Drian has called for an emergency meeting of the U.S.-led coalition of more than 30 countries created to fight Islamic State. France's European affairs minister said next week's EU summit will discuss sanctions on Turkey over its action in Syria.
Russian President Vladimir Putin warned that Islamic State militants could escape from jail as a result of the Turkish offensive, the Interfax news agency reported.