Hundreds of people have died and thousands have fled as Turkish warplanes and artillery continue attacking Kurdish militia targets in northeast Syria.
Turkish warplanes and artillery have hit Kurdish militia targets in northeast Syria on the third day of an offensive that has killed hundreds of people, forced tens of thousands to flee and turned Washington's establishment against President Donald Trump.
Since Mr Trump pulled US troops out of the way following a phone call with Turkish President Tayyip Erdogan, the incursion has opened a major new front in the eight-year-old Syrian civil war and drawn fierce criticism internationally.
"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!" Mr Trump said in a Twitter post on Thursday.
"I hope we can mediate," Mr Trump said when asked about the options by reporters at the White House.
Without elaborating, Mr Trump said the United States was "going to possibly do something very, very tough with respect to sanctions and other financial things" against Turkey.
Meanwhile, France's Europe minister has now said that EU sanctions against Turkey over its anti-Kurd offensive in Syria are "on the table".
"Obviously it's on the table," Amelie de Montchalin told France Inter radio, saying potential reprisals over Turkey's incursion into northeast Syria would be debated during a European Council meeting next week.
A Reuters journalist said a convoy of 20 armoured vehicles carrying Syrian rebels entered Syria from Ceylanpinar on Friday.
Some 120km west, Turkish howitzers resumed shelling near the Syrian town of Tel Abyad, a witness said.
Turkey's Defence Ministry said that in overnight operations the Turkish military and its Syrian rebel allies killed 49 Kurdish militants. It says it has killed 277 militants in total.
The ministry said one Turkish soldier was killed in a clash on Thursday during the offensive, which is targeting the Syrian Democratic Forces (SDF), led by the Kurdish YPG militia.
Kurds said they were resisting the assault. At least 23 fighters with the SDF and six fighters with a Turkish-backed Syrian rebel group had been killed, said the Syrian Observatory for Human Rights, which monitors the war.
NATO member Turkey says the operation is necessary for border security against the YPG militia, which it designates a terrorist group because of ties to militants who have waged a decades-old insurgency in southeast Turkey in which more than 40,000 people have been killed.
Ankara has also said it intends to create a "safe zone" for the return of millions of refugees to Syria.
The SDF have been the main allies of US forces on the ground in the battle against Islamic State since 2014.
They have been holding thousands of captured IS fighters in prisons and tens of thousands of their relatives in detention.
SDF forces were still in control of all prisons with Islamic State captives, a senior US State Department official said in a briefing with reporters on Thursday.
The United States has received a high-level commitment from Turkey on taking responsibility for Islamic State captives but had not yet had detailed discussions, the official said.
US lawmakers have said Mr Trump gave Mr Erdogan the green light to go into Syria but the official disputed that.
"We gave them a very clear red light, I've been involved in those red lights and I know the president did that on Sunday," the official said.
The SDF said Turkish airstrikes and shelling had killed nine civilians. In apparent retaliation by Kurdish-led forces, six people including a nine-month-old baby were killed by mortar fire into Turkish towns, Turkish officials said.
The International Rescue Committee aid group said 64,000 people in Syria have fled since the campaign began.
According to the Observatory, which relies on a network of sources in Syria, at least 29 SDF fighters and 10 civilians have been killed since the start of Turkey's assault.
On the other side of the border, five people including a baby were reportedly killed and dozens injured in Kurdish shelling on Turkish border towns.
AFP correspondents saw fighters crossing into Syria in dozens of vehicles.
Turkey, which has carried out two previous offensives into Syria since the start of the conflict, relies heavily on Syrian proxy fighters, many of who once battled President Bashar al-Assad's troops.