UN humanitarian chief Valerie Amos said mediator Lakhdar Brahimi would press the government and opposition on these humanitarian issues at separate meetings a day after an international conference on Syria.
The two sides, meeting for the first time on Wednesday, vented their mutual hostility but Mr Brahimi said the enemies may be ready to discuss prisoner swaps, local ceasefires and humanitarian aid.
Baroness Amos said: "I have discussed this with Mr Brahimi and he'll continue to push this. Political negotiations can take a very long time and, as we saw yesterday, there are big differences between the parties.
"But if we are able to get a major push on getting into these communities, it would make a significant difference."
She said it was crucial to gain access to some 250,000 people trapped in besieged communities, many of them in Aleppo, Homs and near Damascus, who have been out of reach for many months.
Some say they have been reduced to eating grass in order to avoid starvation.
Another 2.5 million people are in "hard-to-reach" areas, having received UN relief supplies just once or so, Baroness Amos said.
She added: "The key issue for us is that control of communities shifts all the time. We want to really take advantage when there is a sense we can move very quickly to try to do that."
Baroness Amos said there had been little improvement since world powers called unanimously in October for both sides to grant greater access for aid workers and convoys.
A food aid delivery reached Yarmouk, a suburb of Damascus besieged by forces of President Bashar al Assad, on January 18 and was badly needed after months of isolation but not enough for thousands of trapped civilians who are malnourished and without medicine.
Some 50 of 400 planned UN food parcels were delivered in the Yarmouk Palestinian district at the time which Baroness Amos said was a "tiny bit of progress".
Russia, which is Syria's main arms supplier, has been a "strong advocate" in nudging Syrian authorities to grant visas for UN aid workers, she said.
But administrative measures are not enough and there were only three UN aid convoys in December, after the government rejected three and safety guarantees were not in place for two others. Nine convoys delivered goods in November.
UN officials are looking at options for delivering more aid and would discuss them with influential countries at their next high-level meeting set for next month in Rome.
When asked whether humanitarian corridors were under consideration, Baroness Amos said they have proved "notoriously difficult".
She added: "But there are truces, pauses, and reconciliation talks going on in some areas locally."
After yesterday's discussions with Mr Brahimi, the two sides are due to meet again today but Burhan Ghalyoun, a member of the Western-backed Syrian National Coalition, said he expected little from what were intended to be the first face-to-face negotiations between Mr Assad's representatives and members of the rebellion.