US military vehicles and Kurdish fighters from the People’s Protection Units (YPG) drive in the town of Darbasiya next to the Turkish border, Syria April 28, 2017.

A commander of the Syrian Kurdish YPG militia said on Friday US forces would begin monitoring the situation along the Syria-Turkey frontier after cross-border fire between the Turkish military and YPG this week.

The monitoring had not yet begun, but the forces would report to senior US commanders, Sharvan Kobani told Reuters after meeting US military officials in the town of Darbasiya next to the Turkish border.

The officials had toured Darbasiya which was hit by Turkish artillery fire earlier in the week. Turkish warplanes carried out air strikes against Kurdish militants in northeastern Syria and Iraq’s Sinjar region on Tuesday in an unprecedented bombardment of groups linked to the PKK, which is fighting an insurgency against Ankara in Turkey’s southeast.

Those attacks killed nearly 30 YPG fighters and officials, a monitoring group reported. Since Tuesday the YPG and Turkish forces have traded artillery fire along the Syria-Turkey border.

Turkey’s bombardment of YPG positions complicates the US-backed fight against ISIS in Syria, where the YPG has been a crucial partner on the ground for Washington.

The YPG is a key component of the Syria Democratic Forces (SDF), a US-backed alliance of Arab and Kurdish fighting groups involved in a campaign to drive ISIS out of its Syria stronghold, Raqqa.

US NATO ally Turkey views the YPG and other PKK-affiliated groups as terrorists. Pentagon spokesman Captain Jeff Davis said on Friday US troops were deployed along the border.

“We continue to urge all the parties involved to focus on the common enemy which is ISIS,” he told reporters.

Hundreds of US troops are deployed on the ground in Syria to support the Raqqa offensive.

US military vehicles drive in the town of Darbasiya next to the Turkish border, Syria April 28, 2017.

US military vehicles drive in the town of Darbasiya next to the Turkish border, Syria April 28, 2017.

US military vehicles drive in the town of Darbasiya next to the Turkish border, Syria April 28, 2017.