The United Nations said fighting around Syria's capital has cut off 300,000 people from humanitarian assistance and pauses in the conflict are needed to allow aid convoys to get to the area.

Fighting in and around Damascus has intensified in recent days after surprise attacks by rebel fighters in the northeastern parts of the city.

"They are totally dependent on our supplies. Starvation will be just around the corner unless we get there in the coming weeks," Jan Egeland, UN humanitarian adviser on Syria, told Reuters news agency on Thursday.

Egeland said the besieged areas of Douma and Kafr Batna in rural Damascus have not received UN supplies since last year.

"The increase in the fighting has disastrous effects on the civilian population," Egeland said. "They haven't had any supplies by the UN since October in Douma, and in the Kafr Batna area not since June of last year."

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Egeland's comments came as UN-sponsored talks were due to resume in Geneva on Friday, with little hope of a breakthrough or concessions from either the government side or rebels.

Rebels clashed with the Syrian army on the edge of the city centre in the Jobar district for a fifth day on Thursday. Near Hama, rebels also made advances against the Syrian army overnight and fighting continued, UK-based Syrian Observatory for Human Rights said.

The Syrian government has not given the green light for convoys and armed groups have not guaranteed their security, meaning no aid can go in, Egeland said.

He also said that the UN is hoping to send an aid convoy on Friday to Wadi Barada, a valley outside of Damascus where fighting raged at the beginning of the year.

Food had reached the besieged town of Madaya last week, but sniping by fighters surrounding the town meant it could not be distributed, the UN official said.

A report, published last week by Physicians for Human Rights (PHR), found the Syrian government "deliberately" restricted humanitarian access to besieged populations.

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Last year, the government agreed to a two-step aid approval process on humanitarian assistance. It streamlined a previous eight-step procedure, but PHR said the process has failed.

"As the conflict enters its seventh year, Syrian authorities continue to deliberately and illegally manipulate UN humanitarian access, arbitrarily limiting, restricting and denying aid deliveries in order to ensure the continued suffering of besieged populations," the report said.

US Secretary of State Rex Tillerson said on Wednesday that the US would set up "interim zones of stability" to help refugees return home in the next phase in the fight against Islamic State of Iraq and the Levant (ISIL, also known as ISIS) and al-Qaeda in Syria and Iraq.

Tillerson did not clarify where the zone would be set up.

"The United States will increase our pressure on ISIS and al-Qaeda and will work to establish interim zones of stability, through ceasefires, to allow refugees to return home," the top US diplomat said, addressing a meeting of 68 countries gathered in Washington, DC, to discuss the fight to defeat ISIL.

Egeland said any such proposal would need to be carefully studied, adding all returns to Syria had to be voluntary, protected and assisted.

"All zones have to be a net plus for the civilian population. Very often safe zones have created as many problems as they have solved, but it depends on the proposal and how it is executed," he said.

"We'd be interested to discuss this further with the US and others."