Stephen O'Brien told the UN Security Council that last year the United Nations made 113 requests to the Syrian government for approval of inter-agency aid convoys, but only 10% were able to deliver assistance.

Another 10% were approved in principle by the Syrian government, but could not proceed due to a lack of final approval, insecurity or no deal on safe passage, while the UN put 3% on hold due to insecurity.

O'Brien said the remaining 75% of requests went unanswered.

"Such inaction is simply unacceptable," he said. "The impact on the ground is tangible: in 2013, we reached some 2.9 million people through the inter-agency convoy mechanism, but only 620,000 (in 2015)."

Syrian refugees stand in line as they wait for aid packages at Al Zaatari refugee camp in the Jordanian city of Mafraq, near the border with Syria, January 20, 2016.
"More and more people are slipping out of our reach every day as the conflict intensifies and battle lines tighten," O'Brien said.

In total, the UN said 13.5 million people in Syria need humanitarian aid, up from 1.3 million from 2014.

"Even with the worsening situation and continued access challenges, humanitarian workers in Syria continue to stay and deliver aid often at great personal risk," O'Brien said.

He said that in 2015 food was delivered to nearly 6 million people a month; health aid to almost 16 million people; water, sanitation and hygiene support to 6.7 million; and basic household items to 4.8 million.

Syria UN mediator Staffan de Mistura hopes to convene talks on January 29 on ending the civil war, but those plans appeared in doubt after the opposition said it would not show up unless attacks on civilian areas stopped first.

The civil war was sparked by a Syrian government crackdown on a pro-democracy movement in early 2011. Islamic State militants have used the chaos to seize territory in Syria and Iraq, and some 4.3 million Syrians have fled the country.