About 800 UN lorries cross into rebel-held Idlib province each month carrying aid for 2.4 million civilians.
The UN Security Council has reauthorised cross-border deliveries of aid to rebel-held Syria for six months.
Western powers bowed to a demand from Russia, a staunch ally of the Syrian government, which vetoed their proposal for a one-year extension last week.
The mandate for deliveries via Turkey expired on Sunday, prompting the UN aid chief to warn that "people will die".
Some 800 lorries cross into the north-western province of Idlib every month with aid for 2.4 million civilians.
Almost double that number of people, mostly women and children, are trapped in the last stronghold of the jihadist and Turkish-backed rebel groups opposed to President Bashar al-Assad after 11 years of brutal conflict.
An estimated 70% of the civilians already do not have enough to eat at a time when food prices are continuing to rise sharply and one in three children are undernourished, according to the UN.
More than 1.7 million internally displaced people are also having to live in dire conditions in tented camps and informal settlements with limited access to water, sanitation and healthcare.
The region also regularly sees deadly clashes, air strikes and shelling despite a ceasefire brokered by Russia and Turkey in 2020 to halt a government assault.
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More than 56,000 lorries carrying food, medicine, shelter and other live-saving aid have gone into areas of Syria outside the government's control since 2014, when the Security Council authorised the UN's first such cross-border humanitarian operation.
In 2020, Russia and China used their veto powers as permanent council members to stop UN deliveries via Jordan and Iraq, as well as one of the two Turkish border points.
They argued that the operation violated Syria's sovereignty and territorial integrity, and that the aid should cross the front lines via government-controlled territory.
That move meant people living in the Kurdish-controlled north-eastern Syria had to start relying on shipments via Damascus and left the Bab al-Hawa crossing as the UN's only option to access Idlib.