The UN's humanitarian chief has called on Syria to urgently allow fuel imports for aid deliveries and give access to 10 more aid agencies.
Valerie Amos met government officials in the capital, Damascus, to discuss the growing humanitarian crisis.
UN workers also needed to freely move into opposition-controlled areas to step up their aid efforts, she said.
More than 2.5 million people are in need of humanitarian assistance, according to UN estimates.
In addition, more than 500,000 Syrians have fled their homes to neighbouring countries.
At least 20,000 people, mostly civilians, have died since the conflict began 21 months ago.
'Greater rebel contact'
Ms Amos, who met Syrian Foreign Minister Walid Muallem and other officials in Damascus on Saturday, said fuel shortages and lack of staff were hindering essential humanitarian support across the country.
"I also informed the government that we will be making greater contacts with the opposition inside of Syria itself," she told reporters on Tuesday after briefing the UN Security Council on her Syria trip.
"If we are truly to make use of the access that the government says we can have, it means that we have to cross lines, we have to move from government-controlled areas to disputed areas to opposition-controlled areas."
Ms Amos described the security situation in Syria as "volatile", adding that she could hear shelling in Damascus all day long.
Earlier this month, the UN had said it was pulling "all non-essential international staff" out of the country.
Meanwhile all humanitarian missions outside Damascus would be halted for the time being.
In a separate development, rebel fighters claim they have taken full control of the Yarmouk refugee camp in southern Damascus, which houses Palestinians as well as Syrians displaced by fighting.
The claim could not be independently verified.
Earlier on Monday, clashes were reported in the camp between Palestinians from the pro-Assad Popular Front for the Liberation of Palestine-General Command (PFLP-GC) and rebel fighters.
The violence occurred a day after activists said fighter jets had bombed Yarmouk, killing at least eight people sheltering in a mosque.
Ms Amos said the situation in the camp was chaotic at the weekend, and that the UN could not confirm the number of people killed during the fighting.