UNITED NATIONS (Reuters) - The United Nations, backed by the United States, Britain and other powers, urged the Syrian government on Friday (June 3) to end all sieges and allow UN airdrops of aid to hundreds of thousands of people trapped across conflict-torn Syria.

Nearly 600,000 people are besieged in 19 different areas in Syria, according to the UN, with two-thirds trapped by government forces and the rest by armed opposition groups and Islamic State of Iraq and Syria militants.

UN aid chief Stephen O'Brien told the Security Council the world body would on Sunday ask Syria to approve airdrops or airlifts of aid into besieged areas where only partial or no land access had previously been granted, said French UN Ambassador Francois Delattre, president of the council for June.

"I told the council that the operating space for humanitarian actors is shrinking as violence and attacks across Syria increase," O'Brien said in an statement.

"We need the consent of the Syrian government and all necessary security guarantees, in order to conduct airdrops."

Last month members of the International Syria Support Group (ISSG), which includes Russia and the United States, agreed that the UN World Food Programme should airdrop aid to Syria's besieged communities from June 1 if land access was denied.

O'Brien said the UN only reached two of the besieged areas by land last month, representing some 20,000 people, or 3.4 per cent of Syria's total besieged population.

"The Security Council and the rest of the UN, the ISSG, and international community must be prepared for air drops if the regime continues its obstruction," US Ambassador Samantha Power said in a statement.

Syrian UN Ambassador Bashar Ja'afari declined to respond when asked if his government would permit airdrops. He said it was "terrorists," not Damascus, preventing aid deliveries.

"If the Syrian government did not cooperate with the UN with regard to humanitarian aid, millions of Syrians would have died," Ja'afari told reporters.

If Syrian President Bashar Assad's government blocks the airdrops, British Ambassador Matthew Rycroft said his and other governments "will consider further action to ensure that humanitarian aid is delivered." He declined to provide details.

It was not clear why Assad's government would consider agreeing to airlifts for areas where it has blocked land access.

Syria gave the UN and the Red Cross approval on Thursday to send humanitarian aid convoys into at least 11 of the 19 besieged areas during June after the United States and Britain called for airdrops.

Several Western diplomats said the Syrian announcement may be a ploy to deflect discussions on airdrops, noting that Assad's government has a track record of reneging on promises to permit full access to needy people.

Syria's opposition has warned the government may open the door just enough to defuse international pressure before restricting access again.

At least 250,000 people have died in Syria's five-year civil war in Syria, while more than 6.6 million have been internally displaced and another 4.8 million people have fled the country.

Vitaly Churkin, the UN ambassador of Assad's close military ally Russia, suggested Russia was not necessarily opposed to airdrops. "We're open to everything, if it's effective, if it can be done properly and safely," he said.

Russia, like Assad's other ally Iran, is widely seen as having significant influence over the Damascus government.

"Are we always completely successful? No we are not,"Churkin said.