A man carries a woman that survived shelling after what activists said were explosive barrels thrown by forces loyal to Syria's President Bashar al-Assad in the Al-Shaar neighbourhood of Aleppo February 9, 2014. [Photo/Agencies]

On Saturday, gunmen opened fire on civilians, leaving aid workers wounded and two trucks damaged, Erksoussi said, speaking by telephone from Damascus.

Despite the violence, the U.N. High Commissioner for Refugees, António Guterres, said in a statement that the truce showed "that even in the darkest of nights it is possible to offer a glimmer of hope to people in desperate need of assistance."

The Homs cease-fire was arranged by U.N. mediator Lakhdar Brahimi, who urged the warring sides to aid the estimated 2,500 civilians trapped in the ancient, rebel-held quarters known as Old Homs, to build trust during the first face-to-face meetings of government officials and opposition figures in Switzerland last month.

But the truce only took hold after talks ended, and its last day, Monday, now coincides with the beginning of another round of U.N.-mediated negotiations in Switzerland. The Syrian delegation arrived in Switzerland Sunday evening.

Homs was one of the first areas to rise up against Assad in 2011 and has been particularly hard hit by the war. Over the past year, the government regained control over most of the city, except for neighborhoods in the historic center.

Meanwhile, in the northern city of Aleppo, Syrian government aircraft dropped makeshift barrel bombs on a series of rebel-held districts, including one that killed more than 15 people in the neighborhood of Haydariyeh, said the activist group, the Aleppo Media Center. The bombs, crude weapons packed with explosives, fuel and metal, set nine vehicles ablaze, including some carrying civilians fleeing the area.

The bombings are part of a weekslong campaign by Assad's forces to wrest control of Aleppo, parts of which were seized by rebels in mid-2012.

The Syrian uprising began with largely peaceful protests but gradually evolved into an increasingly sectarian civil war pitting predominantly Sunni Muslim rebels against Assad's government, which is dominated by Alawites, an offshoot of Shiite Islam.

Syria's many minorities have thrown their support behind Assad, fearing for their fates should hard-line rebels come to power.

On Sunday, extremist rebels killed at least two dozen Alawite gunmen defending their central village of Maan, the Syrian Observatory said. The Syrian state news agency said a "massacre" had occurred, but provided no further details. A video uploaded by rebel sympathizers showed at least one man killed, and bearded, grinning gunmen looting village homes. The Observatory said women and children had been evacuated before the gunmen entered.

Meanwhile, the Qatar-based broadcaster al-Jazeera aired what it said was new footage of a dozen Syrian nuns who have been held captive by rebels since December.

Rebels seized the nuns and at least three other women from the Greek Orthodox Mar Takla convent when fighters overran Maaloula, a mainly Christian village north of Damascus. The group, who work in the convent's orphanage, was taken to the nearby rebel-held town of Yabroud.

An activist from the area who uses the name Amer, said rebels belonging to the al-Qaida-linked Nusra Front were holding the nuns. He said Qatar officials were trying to negotiate their release and that the video was likely issued to prove to mediators that the women were in good health