Syrian military aircraft dropped barrels bombs on rebel-held areas in the northern city of Aleppo on Saturday, killing at least 15, as a truce broke down between government troops and opposition fighters in the central city of Homs and halted the entry of aid to areas besieged over a year, activists said.

The Aleppo bombings are a key part of a weeks-long campaign by President Bashar Assad's forces to wrest control of the city, parts of which were seized by rebels in mid-2012.

Activists say the massive barrel bombs often clear the way for a government advance. But the crude weapons — cylinders packed with explosives and shrapnel dropped usually from helicopters — cannot be targeted precisely and they have killed hundreds of civilians.

At least five such bombs exploded on roads in the adjacent areas of Masaken Hanano and Haidariyeh, said Mohammed Wissam of the Aleppo Media Center. The activist collective, the Local Coordination Committees, also reported the bombings.

Wissam said four people were killed in Masaken Hanano and another 11 were killed in Haidariyeh.

Video footage of one incident started with a thundering explosion followed by a column of smoke billowing to the sky. Men rushed about a damaged building, pulling out smashed bodies. They carried them on makeshift stretchers of cardboard and blankets, laying the dead on a sidewalk. One man placed a severed limb next to a sprawled body. Men rushed to an ambulance carrying a white sheet laden with body parts; they said they had collected four bodies.

The videos could not be independently authenticated but appeared genuine and corresponded with Associated Press reporting of the event.

Thousands of civilians have fled other pummeled rebel-held areas, said activist Wissam. Their numbers add to the millions who have been displaced during Syria's three-year uprising that began as largely peaceful demonstrations in March 2011.

Also Saturday, the Britain-based Syrian Observatory for Human Rights said they had counted 16 people killed, including five children and a woman, in a government-controlled part of Aleppo. The incidence occurred on Friday, said the Observatory, which relies on a network of activists on the ground.

Armed rebels often fire mortar rounds and other explosives toward rival neighborhoods.

Syrian opposition activists in exile and government officials are expected to meet Feb. 10 in Switzerland to continue tense negotiations over how to end their country's civil war.

As a goodwill gesture, Syrian government forces agreed to partially lift a year-long siege they have maintained on rebel-held blockaded areas of the central city of Homs. They reached a three-day truce with rebels, allowing the evacuation of civilians and the bringing in of food shipments.

But aid that was expected to enter on Saturday was halted after fighting broke out.

A Homs activist who identified himself as Samer al-Homsy said Syrian government forces fired 11 rockets toward the rebel-held Hamidiyeh quarter of the city, halting the shipments.

The Homs governor said rebels fired mortar rounds at a nearby police station. Governor Talal Barazzi spoke to the Syrian state news agency.

On Friday, 83 children, women and elderly people on wheelchairs were evacuated from besieged neighborhoods of Homs, according to numbers issued by the United Nations.

Homs, one of the first areas to rise up against Assad in 2011, has been particularly hard hit. Over the past year, the government has regained control over much of the city, except for a few neighborhoods in the historic center, where rebels are holed up.

UNofficial Khaled al-Masri said they had positioned "food, medical and other basic supplies for immediate delivery."