UN agencies said Tuesday they stood ready to truck aid to besieged rebel-held districts of the city of Homs after a deal at Syrian peace talks, but still needed a green light.

"All the trucks are ready to go. They are loaded, and they are in the UN hub in Homs," World Food Programme spokeswoman Elisabeth Byrs told reporters in Geneva.

"We need to get access, and once access is granted, we'll go," she said, adding that agency aims to truck in enough food to feed 2,500 people for a month.

UN-brokered talks between the Syria's regime and opposition delivered a tangible promise on the humanitarian front Sunday, as UN mediator Lakhdar Brahimi said the regime agreed to allow women and children safe passage from besieged areas of Homs.

The Old City of Homs has been under siege since June 2012 after rebels there rose up against the regime, with an estimated 3,000 people living with near-daily shelling and the barest of supplies.

There has been no progress so far on an evacuation, which the opposition fears could be tantamount to expelling the population of rebel-held areas.

The opposition says convoys must be allowed to deliver supplies to civilians who want to stay in their embattled community.

The government and rebels are trading barbs over who is failing to guarantee security for convoys, but UN agencies declined to pin blame.

"This of course implies security from all parties involved in this operation. We are a humanitarian agency, so we wait until we get a green light. So far we don't have a green light, so we are on standby," Byrs said.

Supplies just 10 kilometres away

The aid stands achingly close to the Old City.

Marixie Mercado, spokeswoman for the UN children's agency UNICEF, said its warehouse was just 10 kilometres (six miles) away.

"UNICEF has a list of supplies with the government now, for approval," Mercado told reporters in Geneva.

They include medical kits to ward off cholera and polio, plus soap, water-treatment kits and winter clothing.

"All of these are available in our warehouse in Homs and can go in as soon as we get a green light, but at this stage we have no clarity on when that might be," she underlined.

With Homs a headline issue at the Syria talks, Byrs said it was crucial to stay focused on the dire situation nationwide.

"This convoy cannot be a fig leaf," she said.

"That's why we're calling for all parties to the conflict to allow the safe passage of humanitarian assistance to all part of the country."

Byrs said aid agencies had only irregular access to a handful of towns in the Homs region, with food deliveries possible only every three to six months.

Parts of northern and eastern Syria have not been reached for months, affecting 775,000 people, and an estimated 800,000 are missing out on supplies in 40 besieged locations in the rural region around the capital Damascus, she added.