Six UN observers had to be evacuated from a northern Syrian town controlled by the opposition Wednesday, a day after a roadside bomb hit their convoy and left them stranded overnight with rebel forces.

The Associated Press reported that none of the observers was wounded and it was not clear who was behind the attack.

According to Ahmad Fawzi, spokesman for international envoy Kofi Annan, the explosion was preceded by shooting which started as the convoy of observers arrived in the opposition area in the town of Khan Sheikhoun.

“The UN observers were in their cars and heard the shooting but did not witness anyone being killed, nor could they ascertain the direction of the fire,” Fawzi told AP. “At the same time, the bomb exploded near one of the vehicles, damaging the hood.”

Fawzi said people who had gathered around the observers ducked behind the vehicles, and according to one observer apparently some people were injured when the bomb exploded.

When the shooting subsided, he said, the observers left their vehicles and proceeded on foot to a Free Syrian Army location where they spent the night.

The UN said the team was treated well during their stay with rebels and returned to their base in Hama on Wednesday.

The presence of United Nations observers in Syria has done little to stem the violence in the country, as the uprising against President Bashar Assad continues. An agreement signed with Assad by Annan has had little more effect than an earlier deal signed with the Arab League.

Annan’s peace plan a “ceasefire” that began on April 12, but neither side has ended the firing.

In his first interview since December, Assad insisted on Wednesday his regime is fighting back against foreign mercenaries who want to overthrow him and not innocent Syrians aspiring for democracy.

 “There are foreign mercenaries, some of them still alive,” Assad told the Russian state news channelRossiya-24. “They are being detained and we are preparing to show them to the world.”