DAMASCUS (AFP) ― Syrian rebels launched fierce assaults on government troops in several parts of the country on Sunday, as a U.N. envoy urged the leader of the opposition to seek dialogue with the Damascus regime.

There were reports of tension between rival rebel groups in the rebel-held northwest of the country, where witnesses say local fighters faced off with a hardline Islamist group.

The Syrian Observatory for Human Rights, which collects reports from a network of activists and medics in civilian and military hospitals on the ground, said at least 50 people were killed in violence across Syria on Sunday.

U.N.-Arab League peace envoy Lakhdar Brahimi met Syrian opposition leader Ahmed Moaz al-Khatib Sunday in the Egyptian capital Cairo on Sunday and urged him to keep seeking dialogue with the Damascus government, the U.N. said.

Brahimi repeated his backing for Khatib’s push for talks and “encouraged the Coalition to continue in this direction,” said a U.N. statement.

Riad Seif, vice president of the National Coalition, was also present at the meeting.

Khatib said in late January he was prepared to hold direct talks with regime representatives who did not have “blood on their hands,” provided replacing President Bashar al-Assad was on the agenda.

Damascus has said it is open to talks but with no pre-conditions.

On the ground, rebels used tanks to shell Brigade 113, just north of the eastern city of Deir Ezzor, the Observatory said.

Surrounded for weeks now, Brigade 113 and the besieged military airport of Deir Ezzor are some of the last regime holdouts in the city, part of a province largely held by the rebels.

The opposition Deir Ezzor press network, a grassroots group of activists reported increasingly desperate conditions there.

“The western countryside is out of water and electricity supplies for the eleventh day in a row due to the indiscriminate shelling by the regime forces,” it said.