By Tom Miles

GENEVA (Reuters) - Around 250,000 civilians in Syria's besieged eastern Aleppo have run out of aid supplies but Syria's government and its allies have not agreed safe passage for a relief convoy, a senior U.N. humanitarian official said on Friday.

Air strikes and shelling in the rebel-held east of Syria's largest city has killed dozens this week, a monitoring group says. The bombardment resumed on Tuesday after a four-week pause, part of a wider military escalation by the Syrian government and its allies, including Russia, against insurgents.

U.N. humanitarian adviser Jan Egeland said civilians trapped by the siege were out of food and medical stocks and bracing themselves for an increasingly fierce attack.

"My understanding is that virtually all warehouses are now empty and tens of thousands of families are running out of food and all other supplies," Egeland told Reuters. "So this is a very bleak moment, and we are not talking about a tsunami here, we are talking about a manmade catastrophe from A to Z."

Rebel groups in Aleppo have agreed in principle to a U.N. humanitarian relief plan that allows medical workers, medical supplies and food into eastern Aleppo and enables the evacuation of the sick and wounded, but operational details have yet to be agreed, Egeland said.

Russia has said it is positive in general about the plan but has not given an official green light, he said. At the same time, Egeland said, Moscow had stepped up its air campaign in support of Syrian and allied ground forces.

The United Nations had hoped to send convoys with aid for 1 million Syrians in besieged or hard-to-reach areas this month, but so far not one has reached its destination.

"The needs are exploding and a killer winter is coming to the exhausted and vulnerable Syrian civilians. We have the trucks, we have relief workers that are willing to go, even though it's dangerous, and we have very concrete plans," Egeland said, adding he was angry and frustrated at the "outrageous" situation.

He blamed arbitrary bureaucracy and the insistence of combatants that the convoys use roads known to be insecure or mined. Although the Syrian government was doing most of the besieging and was responsible for most of the obstacles, Egeland said rebel groups also often did little or nothing to help.

The latest aid convoy had been expected to supply the besieged town of Douma, near Damascus, on Thursday.

"The Republican Guard, at the final checkpoint, refused to follow procedures agreed with the government of Syria, which is that sealed trucks – sealed with government forces looking on – should not be opened until they come to their destination."

(Reporting by Tom Miles; Editing by Mark Heinrich and Janet Lawrence)