Cross-border humanitarian aid to Syria remains vital for the lives of millions, UN Secretary General Antonio Guterres said in an internal report on Tuesday.
It comes as a UN authorisation allowing aid into rebel-held areas in the country's north-west without approval from Damascus is up for renewal.
A rare moment of co-operation between the US and Russia in July allowed a six-month extension of activity at Bab Al Hawa, the only border crossing through which aid reaches the rebel stronghold of Idlib province.
The authorisation, renewable once, runs until January 10.
"Cross-border assistance remains lifesaving for millions of people in need in north-west Syria," Mr Guterres said in a confidential document obtained by AFP.
He said more than four million people were in need of crucial assistance across the country.
The US and several European nations believe the UN authorisation for the crossing between Syria and Turkey should renew automatically for an additional six months, without the need for a new vote.
But Russia, a key ally to the Damascus regime, has previously opposed the move, invoking Syrian sovereignty.
Moscow has linked any potential extension to Tuesday's report, as well as a possible new vote.
The cross-border mechanism has been operating since 2020 through Bab Al Hawa, after the Russian-imposed removal in 2019 of three other access points in Syria.
In Tuesday's document, the UN chief refers to another project for humanitarian operations, this time across the front lines, to reach Idlib.
"If implemented, this plan will make operations across the front lines more predictable and effective," Guterres said.
However, he insisted upon the importance of the Bab Al Hawa crossing.
"At this point such cross-line convoys, even if deployed regularly, could not replicate the size and scope of the cross-border operation," he said.
Mr Guterres said some 4.5 million people in Syria need help this winter, up 12 per cent from the previous year, because of the economic crisis and the Covid-19 pandemic.
Only 2.9 per cent of the Syrian population is fully vaccinated, according to the report.
More than three million people live in Idlib province, much of which is controlled by jihadists and allied rebels.
In June, the UN said around 2.4 million people there were in need of humanitarian aid.