By Ayse Dogru

ANKARA

Turkey on Tuesday demanded an explanation from Russia after a terrorist was sighted among participants at the Syria congress in Sochi, Russia, diplomatic sources said.

Turkey's official delegation saw Mihrac Ural, the leader of the THKP-C -- a pro-Syrian regime militia -- at the congress, according to the sources, who asked not to be named due to restrictions on speaking to the media.

The Turkish Foreign Ministry delegation reportedly demanded an explanation from Russia’s Defense and Foreign Ministries.

The delegation expressed their discomfort at the terrorist’s presence at the meeting in the Russian Black Sea city, as he was not included on the list of invited delegates given to Turkey.

In response, Russian officials said they would immediately look into the matter and report back, the sources added.

Turkey blame Ural for a spate of attacks in May 2013 in the town of Reyhanli in Turkey’s southern Hatay province that left 52 people dead.

The guarantor countries in the Syrian peace process -- Russia, Iran and Turkey -- had agreed to hold talks in Sochi on Monday and Tuesday.

Turkey had repeatedly insisted that while all sectors of Syrian society would be included in the congress, no terrorists or terrorist groups would be allowed.

Syria has been locked in a vicious civil war since early 2011, when the Assad regime cracked down on pro-democracy protests with unexpected ferocity.

Since then, hundreds of thousands of people have been killed and more than 10 million others displaced, according to UN officials.