Ankara: US Vice President Mike Pence arrived in Turkey on Thursday to push President Recep Tayyip Erdogan for a ceasefire in Syria after Ankara rebuffed international pressure to halt its deadly offensive against Kurdish forces.
But Erdogan vowed on Wednesday that Turkey's operation -- which has been facilitated by the withdrawal of US troops from northern Syria -- would continue.
That came as an extraordinary letter from Trump emerged in which he warned Erdogan: "Don't be a fool".
Sent the day Turkey launched its incursion into northeastern Syria, Trump said history risked branding him a "devil".
Pence is accompanied by US Secretary of State Mike Pompeo and other officials.
Days after US troops abruptly began withdrawing, Turkish soldiers and their Syrian rebel proxies gained ground in Ras al-Ain, a key border town where Kurdish fighters had been putting up stiff resistance, the Syrian Observatory for Human Rights said.
Turkish forces and the mostly Arab and Turkmen former rebels they use as a ground force had "taken about half of the town" by Thursday morning, Rami Abdel Rahman, head of the Britain-based war monitor, said.
The Observatory said more than 300,000 civilians were displaced within Syria since the start of the assault, and labelled it one of the largest upheavals since Syria's civil war began in 2011.
The monitor said nearly 500 people have been killed including dozens of civilians, the majority on the Kurdish side.
On Wednesday, Kurdish forces struck a desperate deal with Damascus and stepped aside to allow Syrian regime troops and allied Russian soldiers enter the border town of Kobane, according to the Observatory.
Kobane is a highly symbolic town for Syria's Kurds, whose forces had in 2015 wrested it from the Islamic State (IS) group in an epic battle backed by the US-led coalition.
The Turkish operation, now in its second week, has triggered a flurry of diplomacy among major powers.
Facing a barrage of criticism in Washington for abandoning the Kurds, Trump has imposed sanctions on three Turkish ministers and raised tariffs on its steel industry.
Pence's office said the US would pursue "punishing economic sanctions" unless there was "an immediate ceasefire".
In the missive to Erdogan dated October 9 - whose authenticity was confirmed to AFP by the White House - Trump wrote: "You don't want to be responsible for slaughtering thousands of people, and I don't want to be responsible for destroying the Turkish economy -- and I will." Kremlin spokesman Dmitry Peskov pointed out the "unusual" nature of the letter, saying it was "not often that such language is found in exchanges between leaders".
But Erdogan told the Turkish parliament that the only way to solve Syria's problems was for the Kurdish forces to "lay down their arms... destroy all their traps and get out of the safe zone that we have designated".
Trump again dismissed the idea that pulling out 1,000 troops - practically the entire US contingent in the region - had been a betrayal of Kurdish militants who bore the brunt of the fight against IS in recent years.
Ankara says Syria's main Kurdish force, the People's Protection Units (YPG), is a "terrorist" offshoot of the outlawed Kurdistan Workers' Party (PKK), which has waged a bloody insurgency inside Turkey since 1984.
Moscow has stepped into the void left by the US withdrawal, deploying patrols to prevent clashes between Syrian and Turkish forces.
The Kremlin and the Turkish presidency said Erdogan would meet his Russian counterpart Vladimir Putin next week, as both sides seek to prevent a war between Turkey and Syria.
The Turkish government can count on widespread support for its operation at home, where a decades-long bloody insurgency by Kurdish militants has killed tens of thousands of people.
But Western powers fear it will endanger the fight against IS. Thousands of IS prisoners are held in Kurdish-run camps in the region.
Europe has taken an increasingly tough line with Turkey and several countries, including Britain, France and Germany, have imposed arms embargoes on Ankara over the operation.
The UN Security Council warned in a unanimously adopted statement of a risk of "dispersion" of jihadist prisoners, but stopped short of calling for an end to Turkey's offensive.
The Kurdish-led Syrian Democratic Forces said Wednesday they were "freezing" operations against IS.
The force's head Mazloum Abdi told Kurdish television channel Ronahi it would make do with "defensive" operations against the group, which maintains sleeper cells and a presence in Syria's vast desert despite its territorial defeat.
Since launching their assault on October 9, Turkey and its Syrian proxies have secured more than 100 kilometres of the border, but Ras al-Ain has held out.
Erdogan wants to create a buffer zone stretching 30 kilometres from the border into Syrian territory.
He wants to destroy Kurdish hopes of an autonomous enclave that Turkey fears could serve as a launchpad for attacks on its soil, and to resettle some of the 3.6 million Syrian refugees Ankara is hosting.