BEIRUT — A suicide bomber blew himself up at the gates of a Syrian prison Thursday and rebels stormed in behind him, freeing hundreds of inmates as part of an offensive aimed at capturing key government symbols around the northern city of Aleppo.

Government forces, meanwhile, dropped crude "barrel bombs" in deadly airstrikes as both sides escalated their fight for the strategic city ahead of a second round of peace talks set for next week. Opposition leaders threatened to suspend the talks over the barrel bombings.

In the past six days alone, the makeshift weapons — containers packed with explosives, fuel and scrap metal — have killed more than 250 people in Aleppo, including 73 children, according to the Britain-based Syrian Observatory for Human Rights.

They include at least 11 who died Thursday — six of them from the same family — in the opposition-held neighborhood of Masaken Hanano.

Videos uploaded by activists showed the aftermath, including men weeping amid ravaged buildings and corpses covered with blankets on the pavement.

"Be careful. There's a corpse under your feet. .. It's a child!" someone shouted. The videos were consistent with reporting by The Associated Press.

The rebels in Aleppo declared a push to seize the city's central prison and the Kweiras military air base to the east. Opposition fighters have been trying to capture the installations for months.

Thursday's offensive began when a Chechen suicide bomber from the al-Qaida-linked Nusra Front attacked the prison gates, according to the Observatory. Rebel fighters then managed to gain control of large parts of the compound. By evening, heavy clashes between the rebels and soldiers were raging inside.

The Observatory and other activists said the rebels freed several hundred prisoners.

State-run Syrian television said the army foiled an attempt by "terrorist groups" to attack the prison.

Rebels have been besieging the prison, estimated to have 4,000 inmates, for almost a year. They have rammed suicide car bombs into the front gates twice, lobbed shells into the compound and battled frequently with the hundreds of guards and troops holed up inside.

The nearly 3-year-old uprising against President Bashar Assad has left more than 130,000 people dead and forced more than 2.3 million to seek refuge abroad.

The Syrian government has not said whether it plans to take part in the proposed new round of U.N.-hosted peace negotiations in Geneva, although its chief ally, Russia, expressed confidence earlier this week that the government would indeed return.

Michel Kilo, a senior opposition figure and member of the negotiating team, said he and other figures were discussing suspending the peace talks until the government halts its "hysterical" use of barrel bombs.

"The Syrian regime is not interested in a political solution. ... They see the talks as an opportunity to advance on the ground," he told the AP.

The Syrian government announced Thursday that it has reached an agreement with the United Nations to secure the evacuation of hundreds of trapped civilians from besieged parts of the city of Homs and allow humanitarian aid to be delivered to those left behind.

U.N. deputy spokesman Farhan Haq welcomed the news.

"The United Nations and humanitarian partners had prepositioned food, medical and other basic supplies on the outskirts of Homs ready for immediate delivery as soon as the green light was given by the parties for safe passage," he said in New York.