Aid operations were due to start in besieged areas of Homs Friday, a US official said, as Syria and the UN agreed to hold a "humanitarian pause" on fighting in the devastated city.
The agreement came as an NGO said more than 250 people have died in regime barrel-bomb attacks on Aleppo, with UN Secretary General Ban Ki-moon condemning the assault.
Also in northern Syria, clashes between rebels and troops raged inside Aleppo's central prison, after Islamist fighters assaulted it and freed hundreds of prisoners, the Syrian Observatory for Human Rights said.
A week after the situation in besieged districts of Homs was discussed at peace talks in Geneva, state news agency SANA said agreement has been reached to allow aid in and safe passage out for civilians.
"Homs Governor Talal al-Barazi and UN resident coordinator Yaacoub El Hillo have reached an agreement securing the exit of innocent civilians from the Old City and the entrance of humanitarian assistance for civilians who choose to stay," it said.
The UN announced there would be a "humanitarian pause", with spokesman Farhan Haq citing UN Under Secretary General for Humanitarian Affairs and Emergency Relief Valerie Amos.
The deal clears the way for delivering lifesaving supplies for about 2,500 trapped civilians, with activists saying people have survived on little more than olives for weeks.
SANA said "the relevant Syrian authorities will implement the deal by providing the necessary humanitarian assistance".
State Department spokeswoman Jen Psaki said: "We understand the operations will begin tomorrow, Friday morning, and will include a local humanitarian pause while the evacuations take place and while the food and other humanitarian assistance is delivered."
Rebel-held districts in Homs have come under nearly daily shelling since the army blockaded them in June 2012.
Among the besieged residents are at least 1,200 women, children and elderly people, the Britain-based Observatory says.
In Geneva, the Office of the High Commissioner for Human Rights said food and supplies had been placed on the outskirts of Homs but they would not be delivered until safety is assured.
"They're not going to travel by night, but the agreement for delivery is there, and that is what we are welcoming," OHCHR spokesman Jens Laerke said.
"You may only hear about the actual delivery when it has taken place. And that is simply to ensure the safety of our staff."
In Aleppo, meanwhile, the Syrian Observatory said more than 250 people have been killed in six days of regime barrel-bomb attacks, as a coalition of rebels announced a new military operation in the province.
"The Secretary-General is following with deep concern the continued armed escalation in Syria, most deplorably the ongoing aerial attacks and the use of 'barrel bombs' to brutal, devastating effect in populated areas," said Ban's spokesman, Martin Nesirky.
"He condemns once again the indiscriminate use of any weapon against civilians, in contravention of obligations under international humanitarian and human rights law," Nesirky said. "All civilians must be protected in any situation."
The Islamic Front, which groups tens of thousands of rebels, and the Al-Qaeda franchise Al-Nusra Front announced a joint operation dubbed "Truthful Promise Approaches", a reference to a passage in the Koran.
The announcement comes as the army seeks to take territory in the rebel-held eastern part of Aleppo city.
That ground campaign has been accompanied by daily aerial attacks since Saturday by barrel bombs dropped from helicopters.
These attacks have killed at least 257 people, including 76 children, and wounded hundreds more, the Observatory said.
Widely condemned as indiscriminate, the bombing has sparked a mass exodus from the worst-hit neighbourhoods in the east of the city.
Just outside the city, rebels and jihadists assaulted the central prison, freeing hundreds of detainees, the Observatory said.
But the NGO's director Rami Abdel Rahman said the army took back the initiative after Ahrar al-Sham -- part of the Islamic Front -- and Al-Nusra had captured large parts of the sprawling complex.
An Ahrar al-Sham fighter told AFP clashes were ongoing, and state television said the attack was repulsed.
The assault began with a suicide attack by an Al-Nusra fighter at the prison's main entrance.
In New York, the UN Security Council demanded Syria move faster to remove its deadly chemical weapons stockpile and meet a June 30 deadline set for destroying its arsenal.
The 15 member nations "call upon the Syrian Arab Republic to expedite actions to meet its obligations", the council's president for the month, Lithuania's UN ambassador Raimonda Murmokaite told reporters.
The chemical weapons must be transported to the Syrian port of Latakia "in a systematic and sufficiently accelerated manner", he insisted.
President Bashar al-Assad has pledged to eliminate Syria's entire chemical weapons arsenal by the end of June or face sanctions, including the possible use of force.
Less than five percent of the stockpile has been removed from Syria, according to Washington, and Damascus has just missed another key deadline.