Russia on Thursday blocked a U.N. Security Council demand that Syria allow immediate access to thousands of civilians trapped by a government offensive on the city of Homs, diplomats said.

Diplomats said Russia's opposition to the statement proposed by council members Australia and Luxembourg was a new sign of a growing international split over the 26-month old conflict.

The statement, obtained by Agence France Presse, sought "immediate, safe and unhindered access" to Homs, where President Bashar Assad's forces have been joined by fighters from Hizbullah against rebels.

Government warplanes bombed rebel-held neighborhoods of Syria's third city on Thursday, according to Syrian activists.

U.N. leader Ban Ki-moon raised fears this week for an estimated 2,500 civilians that he said were "trapped" in Homs' Old City. U.N. humanitarian agencies say they have medical supplies ready but cannot get in.

Russia, Assad's key international ally, similarly blocked a U.N. Security Council call last month for access to rebel held Qusayr until government forces had taken the key town.

Australia and Luxembourg proposed the new statement in a bid to strengthen calls for access made by Ban and the U.N. Office for the Coordination of Humanitarian Affairs (OCHA).

The proposed statement expresses "grave concern about the estimated 2,500 civilians trapped in Homs."

It calls on the Syrian government "to facilitate immediate, safe and unhindered access, in accordance with the United Nations guiding principles of humanitarian assistance." The statement stresses that medical assistance was particularly urgent in Homs.

It said rebel and government forces should allow civilians to leave Homs, and highlighted "the primary responsibility of the Syrian government in this regard."

Russia asked for a 24 hour delay when the statement was first proposed but when the latest deadline for agreement came up on Thursday morning, put a hold on the declaration by the 15-nation body.

"The Russians have asked for changes," said one U.N. diplomat. "What they are asking means that there is no chance we will agree anything today, even though the situation is now urgent," added a second U.N. diplomat, also speaking on condition of anonymity.

"It is quite clear, this shows that it is impossible for us to reach an agreement on Syria," said a diplomat from a Security Council nation who confirmed the deadlock.

Russia and China have used their powers as permanent members of the Security Council to veto three western proposed resolutions which sought to increase pressure on Assad.

Russia and the United States have said they want to hold an international conference on Syria. But original plans for a meeting in May have been pushed back to at least September and some U.N. diplomats say they now doubt a conference can be convened.

The Syrian National Council, the main opposition group, said Thursday that the fall of Homs could destroy any hopes of a political solution to the war, which Syrian activists say has left more than 100,000 dead.

The government says it has asked the International Committee of the Red Cross to help evacuate civilians from Homs.

The U.N.'s humanitarian agency, OCHA, said on Wednesday however that "no safe passage has been offered by the parties to the conflict."

OCHA said that it had stockpiled medical necessities and food for 40,000 people in various parts of Homs but could not move the supplies.