GENEVA (Reuters) - Turkey could be deemed responsible for summary executions by an affiliated armed group of captured Kurdish fighters and a politician, acts that may amount to war crimes, the United Nations said on Tuesday.

The U.N. human rights office also said it had documented civilian casualties caused by air strikes, ground-based strikes and sniper fire each day since the Turkish offensive began in northeastern Syria nearly a week ago.

Turkey and its Syrian rebel allies launched a military offensive into Kurdish-held parts of the northeast saying it aims to defeat the Kurdish YPG militia which it sees as terrorists for its links to separatists in Turkey.

The Kurdish-led Syrian Democratic Forces (SDF) accused Turkey-backed fighters of killing a Kurdish politician in an ambush on a road in northern Syria on Saturday. A Turkey-backed rebel force denied the killing and said it had not advanced that far.

U.N. human rights spokesman Rupert Colville said video footage appeared to show executions of three Kurdish captives carried out by Ahrar al-Sharqiya fighters, affiliated with Turkey, on the highway between Hasaka and Manbij on Oct 12.

The United Nations had received reports that Hevrin Khalaf, a Kurdish politician, was executed on the same highway by the same group on the same day, he said, adding that summary executions may amount to war crimes.

“Turkey could be deemed responsible as a state for violations by their affiliated groups as long as Turkey exercises effective control of these groups or the operations in the course of which those violations occurred,” Colville told a news briefing.

“We urge Turkish authorities immediately to launch an impartial, transparent and independent investigation and to apprehend those responsible, some of whom should be easily identifiable from the video footage they themselves shared on social media,” he said.

U.N. war crimes investigators would follow up on all incidents, he added.

At least four civilians, including two journalists, were killed and dozens injured when a convoy was hit by a Turkish air strike on the Syrian town of Ras al Ain on Sunday, Colville said.

The U.N. human rights office had reports of alleged air and ground-based strikes on five health facilities by Turkish forces and affiliated groups. It had reports of attacks on civilian infrastructure, including power lines, water supplies and bakeries, he said.

Turkish authorities have reported that 18 civilians were killed in Turkey, including a nine-month-old baby, by cross- border mortar and sniper fire by Kurdish fighters, he said.