WASHINGTON - US Secretary of State Mike Pompeo on Saturday described as "shameful" Russia and China blocking a UN Security Council resolution that would have extended for a year cross-border humanitarian aid to four million Syrians.

"The Russian Federation's and China's veto yesterday of a Security Council resolution that allows for humanitarian aid to reach millions of Syrians is shameful," Pompeo said in a statement.

"To Russia and China, who have chosen to make a political statement by opposing this resolution, you have blood on your hands."

Humanitarian aid currently flows into Syria through UN-designated checkpoints in Turkey and Iraq without the formal permission of the regime in Damascus, but that authority is due to expire on January 10.

Germany, Belgium and Kuwait presented a resolution extending that authority for a year, winning the support of 13 council members but drawing the vetoes of Russia and China.

A competing Russian resolution would have granted a six-month extension while reducing the number of UN crossing points, but it failed to get the minimum nine votes.

Russia, an ally and major supporter of Syrian President Bashar Al-Assad, has used its veto 14 times on Syrian issues since civil war broke out there in 2011.

Russia's ambassador to the United Nations, Vassily Nebenzia, said the latest resolution was "obsolete" because the authorities in Damascus had "retaken control of most" of Syria's territory.

But the UN humanitarian relief department says the aid remains crucial as the situation on the ground has deteriorated and Syria is heading into winter.

Four million Syrians directly benefit from the cross-border aid shipments.

"I am in a state of shock," Kelly Craft, the US ambassador, said after the Russian and Chinese vetoes. "I am deeply and profoundly disappointed."

British envoy Karen Pierce said the Russian veto showed "breathtaking hypocrisy" by Moscow.

The resolution failed just as tens of thousands of civilians have been fleeing the northwestern Idlib region amid heavy bombardments by Assad's Russian-backed government, in the last bastion of the jihadist opposition. -AFP