The UN Security Council united for the first time on a resolution on Syria's humanitarian crisis Saturday, unanimously demanding that President Bashar al-Assad's government and the opposition provide immediate access everywhere in the country to deliver aid to millions of people in desperate need.

The fate of the Western and Arab-backed resolution rested with Russia, Syria's closest ally, and China, another supporter.

They decided to join the rest of the 15-member council in sending a strong message to the Assad government that food, medicine and other essentials must not be blocked to civilians caught in the three-year conflict.

The resolution does not threaten sanctions — Russia insisted that this reference be dropped from the original Western- and Arab-backed text — but it does express the council's intention to take "further steps" if the resolution isn't implemented.

UN Secretary-General Ban Ki-moon told the council immediately after the vote that the resolution "should not have been necessary" because "humanitarian assistance is not something to be negotiated — it is something to be allowed by virtue of international law."

"Half the country's people need urgent assistance," he said. "Host countries need support in caring for more than 2.5 million refugees."

The UN chief said it is "profoundly shocking … that both sides are besieging civilians as a tactic of war."

"Some 200,000 people are under siege in government-controlled areas — and 45,000 in opposition-controlled areas," he said.

'Painfully slow' progress

Russia and China had vetoed three previous resolutions backed by Western nations that would have pressured Assad to end the conflict, which according to activists has killed more than 136,000 people.

Britain's UN Ambassador Mark Lyall Grant said after the vote: "Today the council has finally shown that whatever its political differences over Syria it was not entirely indifferent to the devastating humanitarian crisis."

Syria is in danger of losing a generation of its children. - UN humanitarian chief Valerie Amos

The Security Council did come together in October to approve a weaker presidential statement on the worsening humanitarian crisis in Syria.

But UN humanitarian chief Valerie Amos said last week that the presidential statement has not delivered the results that are critically needed, calling progress on the humanitarian front in the last four months "limited, uneven and painfully slow."

She backed a legally binding council resolution if it has "levers" that lead to change on the ground.

Amos issued a statement Saturday expressing hope that adoption of the resolution, "will facilitate the delivery of aid to people in desperate need in Syria."

"It is also vital that ordinary people, who have been bearing the brunt of the violence, are protected," Amos said. "More than anything the conflict needs to end so that people can begin to rebuild their lives. Syria is in danger of losing a generation of its children. Children are the future. We must protect them."