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 US President Donald Trump.

The incursion, launched after Trump withdrew US 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 armored 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 spokesperson  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.