Syria crisis: UN siege aid 'could start within 24 hours'
The UN says it hopes to start delivering aid to some besieged areas in Syria within the next 24 hours.
The move comes shortly after world powers agreed to push for a cessation of hostilities in a week's time.
Some Syrian cities have been cut off from humanitarian aid for over a year because of fighting. About 13.5 million people are in need of aid, the UN says.
More than 250,000 people have been killed and some 11 million displaced in almost five years of fighting in Syria.
A new UN task force to co-ordinate the distribution of the aid is expected to convene in Geneva later.
"The UN system has been geared to deliver this aid all along, especially to besieged areas, and that's precisely what's going to be discussed today: how to start, and when to start," UN spokesman Ahmad Fawzi said.
"We hope to start as early as tomorrow, immediately after the meeting, decisions will be taken to roll the aid in, especially to besieged areas that need it", he added.
The plan to deliver aid was part of a package of measures agreed by the 17-member International Syria Support Group (ISSG) in Munich on Friday.
The group also agreed to seek a nationwide "cessation of hostilities" in Syria to begin in a week's time in measures.
The halt will not apply to the battle against jihadist groups Islamic State (IS) and al-Nusra Front.
The Syrian government has not yet responded, though a key rebel coalition welcomed the joint announcement by US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov.
"If we see action and implementation on the ground, we will be soon in Geneva," Salim al-Muslat told reporters, referring to UN efforts there to get peace talks between the Syrian government and rebels off the ground.
The announcement comes as the Syrian army, backed by Russian air strikes, advances in Aleppo province.
The move threatens to encircle tens of thousands of civilians in rebel-held parts of the major city of Aleppo.
Planned peace talks between the Syrian government and opposition groups in Geneva collapsed earlier this month.
Major gaps remain: BBC Chief International Correspondent Lyse Doucet
Both Sergei Lavrov and John Kerry admitted, repeatedly, this was only progress on paper. Some diplomats are already saying "it's not worth the paper it's printed on".
There are still major gaps. One of the biggest is that Russia's bombing of Aleppo and what it calls terrorist targets is not included in the possible truce even though its actions are seen by many as strengthening Syrian government forces.
On the issue of delivering desperately needed aid to besieged areas, UN officials say they are determined to seize this new opening.
The next week will confirm whether Syria's government and opposition forces are ready to provide access denied for so long.
It will be a week which tests the commitment of all outside players, as well as Syrians on all sides.
That, in itself, is some progress. But moving towards talks to end Syria's devastating war will still take far more than that.
Mr Kerry admitted the ceasefire plan was "ambitious" and said the real test would be whether the various parties honoured the commitments.
"What we have here are words on paper, what we need to see in the next few days are actions on the ground," he said.
US Defence Secretary Ash Carter told the BBC that Russia was "still way off track on Syria".
"More broadly in Syria they said they would come in and fight ISIL and they didn't, instead they joined the civil war and fuelled the civil war", he said.
NATO Secretary-General Jens Stoltenberg also sounded a similar note of caution.
"Air strikes of Russian planes against different opposition groups in Syria have actually undermined the efforts to reach a negotiated, peaceful solution" he said.
Russia insists it only targets extremist groups within Syria.
What has been agreed?
To try to immediately step up aid deliveries to besieged and hard-to-reach areas in Syria
For a US/Russia-led task force to work to achieve a "cessation of hostilities" across Syria beginning in one week's time
"Cessation of hostilities" will exclude action against so-called Islamic State group, jihadist group al-Nusra Front and other UN-designated terrorist groups
To work towards an eventual ceasefire and implementation of a UN-backed plan for political transition in Syria
Mr Lavrov said there were "reasons to hope we have done a great job today". An earlier proposal from Russia envisaged a truce starting on 1 March.
Syria conflict - key questions
Why is there a war in Syria?
Anti-government protests developed into a civil war that, four years on, has ground to a stalemate, with the Assad government, Islamic State, an array of Syrian rebels and Kurdish fighters all holding territory.
Who is fighting whom?
Government forces concentrated in Damascus and the centre and west of Syria are fighting the jihadists of Islamic State and al-Nusra Front, as well as less numerous so-called "moderate" rebel groups, who are strongest in the north and east. These groups are also battling each other.
How has the world reacted?
Iran, Russia and Lebanon's Hezbollah movement are propping up the Alawite-led Assad government, while Turkey, Saudi Arabia and Qatar back the more moderate Sunni-dominated opposition, along with the US, UK and France. Hezbollah and Iran are believed to have troops and officers on the ground, while a Western-led coalition and Russia are carrying out air strikes.