Syrian rebels launched fierce assaults on government troops in several parts of the country, as a UN 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.
UN-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 UN said.
Brahimi repeated his backing for Khatib's push for talks and "encouraged the Coalition to continue in this direction," said a UN 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.
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"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.
In the northeast province of Raqa, fighters from the jihadist Al-Nusra Front and other rebel battalions took over an army company in Tabqa along the Euphrates river.
They secured a major cache of artillery and ammunition and captured a key checkpoint in the town, the Observatory said.
Rebels kept up their assault on Wadi Deif military base, a major regime holdout in the largely rebel-held northwest province of Idlib. The army retaliated by shelling the nearby town of Maaret al-Numan.
Elsewhere in Idlib province however, witnesses reported growing tension between the hardline Al-Nusra Front fighters and other rebels.
Near Atme, local fighters and Al-Nusra almost started fighting when the Islamists attempted to try a man in an Islamic court for swearing, witnesses told AFP.
Locals kidnapped an Al-Nusra leader, put a grenade in his mouth and cut off his beard, before releasing him a few days later, they added.
The hardline Al-Nusra Front group has claimed the bulk of the deadly suicide bombings that have rocked Syria since its March 2011 uprising against Assad.
It has a number of foreign fighters and is believed to be closely linked to Al-Qaeda in Iraq blacklisted as a terrorist organisation in the US.
In the northern province of Aleppo four soldiers were killed and more than 20 wounded in a bombing by Al-Nusra Front targeting a military building, the Observatory said.
Rebels also attacked an army convoy south of Aleppo city, and a cement plant in the central Hama province killing five guards.
In the capital Damascus a blast blew up near a police station, injuring two civilians, while a mortar hit the nearby Shahbandar Square causing no injuries, the Observatory said.
The United Nations estimates that more than 60,000 people have been killed since the revolt against Assad's regime began in March 2011.