Russia has blocked a British-drafted UN Security Council statement condemning the Syrian government attacks on the city of Aleppo, diplomats said.

It was the second time in a month that Russia objected to a western bid to slam President Bashar al-Assad's air assault against Syria's biggest city that has killed hundreds since December 15.

Aid groups say that Scud missiles and barrels packed with explosives and shrapnel have been dropped on schools, markets and hospitals in Aleppo, killing more than 700 people.

Britain circulated a draft press statement expressing "outrage" at the government attacks on civilians to the other 14 members of the council Tuesday.The statement needs the agreement of all members to be released, and Russia on Wednesday objected, insisting all references to Aleppo be taken out, diplomats said.

"They just want a general text condemning all attacks by all sides in Syria," said one diplomat involved in negotiations.

Russia is a key backer of Assad and has used its power as a permanent council member to veto three legally binding resolutions on Syria since the war started in March 2011.

"The members of the Security Council expressed outrage at the daily use of air strikes by the Syrian government against the city of Aleppo which have left over 700 dead and over 3,000 injured since December 15," said the statement.

It added that the attacks were a "cynical and systematic" disregard of international law banning attacks on civilians.

The US proposed a non-binding statement on Aleppo in December, but Russia blocked it with what Western diplomats called "wrecking amendments" that would have watered it down.

More than 120,000 people have been killed and millions forced from their homes in Syria's nearly three-year conflict.