UNITED NATIONS, United States (AFP) - The UN Security Council on Saturday adopted a unanimous but non-binding resolution calling for humanitarian aid convoys to be allowed access across war-torn Syria, but diplomats immediately voiced doubt about its effectiveness.

Syria's staunch ally Russia, with support from China, has blocked three previous resolutions aimed at pressuring the Damascus regime since the crisis began in March 2011, with an estimated half of all Syrians urgently awaiting immediate help.

But Moscow and Beijing, two of the five permanent Security Council members, did not do so this time, sending a strong message to President Bashar al-Assad, whose regime is accused of serious rights violations in attempting to hold on to power.

The resolution was drafted by Australia, Jordan and Luxembourg and had the backing of Britain, France and the United States, the other permanent Security Council members.

UN Secretary-General Ban Ki Moon welcomed the move, but said the resolution "should not have been necessary".

"Humanitarian assistance is not something to be negotiated; it is something to be allowed by virtue of international law," he said.

"Profoundly shocking to me is 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."

Mr Ban added: "If this resolution is implemented quickly and in good faith, at least some of the suffering can be eased.

"It builds on the presidential statement adopted last year, and strengthens the Council's engagement in protecting civilians and ensuring the delivery of relief."

The humanitarian situation in Syria, where more than 140,000 people have been killed in the nearly three-year war and millions more forced to flee their homes, "continues to deteriorate," added Mr Ban.

However, some diplomats doubt the effectiveness of the resolution in the absence of automatic sanctions should Damascus refuse to let aid convoys have access to all areas, including those hardest-hit.

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

"Civilians continue to bear the brunt of the conflict. They are the daily victims of brutal violence and indiscriminate attacks, including the use of heavy weapons, aerial bombings, mortars and car bombs in population areas.

"There are continued reports of massacres and atrocities throughout the country."