Syrian airstrikes, shelling kill 400 so far this month in Aleppo's rebel areas, activists say.

The Associated Press

Syrian citizens ride in the back of a truck with their belongings after fleeing Yabroud, the last rebel stronghold in Syria's mountainous Qalamoun region, as they drive towards the Lebanese-Syrian border town of Arsal in eastern Lebanon, Thursday, Feb. 13, 2014. Syrian troops pounded Thursday the town of Yabroud the last rebel stronghold in Syria's mountainous Qalamoun region, forcing hundreds to flee into the nearby Lebanese town of Arsal. Backed by Lebanon's Hezbollah fighters, the Syrian army has been on a crushing offensive in the region since early December. (AP Photo/Bilal Hussein)

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By JOHN HEILPRIN and BARBARA SURK, Associated Press

GENEVA (AP) — The United States and Russia promised to try to break the stalemate in Syria peace talks, a U.N. mediator said Thursday, as Syrian activists said government shelling and airstrikes with makeshift barrel bombs killed about 400 people in the country's largest city so far this month.

A second round of peace talks in Geneva has offered a rare opportunity for conversation, but yielded little more than acrimony, with both the Syrian government and opposition signaling they believe negotiations could be over. The violence has escalated on the ground and delegates in Geneva have failed to even agree on an agenda for the talks.

U.N.-Arab League mediator Lakhdar Brahimi said after meeting with senior U.S. and Russian officials Thursday that they pledged to try help.

"They have kindly reaffirmed their support to what we are trying to do and promised that they will help both here and in their capitals and elsewhere to unblock the situation for us because until now we are not making much progress," he told reporters.

He met with U.S. Undersecretary of State Wendy Sherman and Russian Deputy Minister of Foreign Affairs Gennady Gatilov to try to salvage the talks.

"Failure is always staring at us in the face. As far as the U.N. is concerned, we will certainly not leave one stone unturned if there is a possibility to move forward," he said.

The bombings in Aleppo are part of a campaign by President Bashar Assad's forces to wrest control of neighborhoods that were seized by rebels in the northern city since mid-2012.

They come as a cease-fire in the central city of Homs has been extended for three days as of Thursday in order to allow more people to leave besieged rebel-held parts of the city, the Homs governor said. Gov. Talal Barrazi said that as long as there are people who want to leave rebel-held areas in Homs, the truce will be extended.

An official at Barrazi's office said there were no evacuations from Homs on Thursday, adding that officials were working on clearing some 70 men of fighting age who left over the past days.

The official, who spoke on condition of anonymity in line with regulations, said evacuations are expected to resume on Friday. It was the second extension since the truce went into effect last week.

Hundreds of civilians have been evacuated from Homs since Friday when a rare cease-fire went into effect. Aid workers took advantage of the temporary truce that was implemented by the warring sides before the second round of peace talks started in Geneva this week. The cease-fire expired on Wednesday night.

U.N. spokesman Martin Nesirky said the U.N. Office for the Coordination of Humanitarian Affairs, or OCHA, reported that 1,370 people have been evacuated from the Old City of Homs since last Friday and that food, medical supplies and essential household and hygiene items have been delivered for 2,500 people, with enough food for one month.

Khaled Erksoussi, the head of operations with the Syrian Arab Red Crescent told The Associated Press that since Friday, about 1, 500 people have been evacuated from the besieged areas.

Before the aid effort began last Friday, the SARC estimated that up to 3,000 people were trapped in the rebel-held district that had been under government blockade for more than a year, causing hunger and widespread suffering of civilians in Syria's third largest city.

Nesirky also said that over the last two days, aid was also delivered to Bloudan, a tense suburb of Damascus near the border with Lebanon, including food and medicines for 5,000 people.

"Bloudan is only 45 kilometers (28 miles) from Damascus, but it is in a hard-to-reach area because of insecurity," he said. "The humanitarian team reported that it took four hours to travel the last 15 kilometers (9 miles) and more than 20 checkpoints had to be negotiated."