UNITED NATIONS (Reuters) - Russia prevented the U.N. Security Council on Saturday from criticizing statements by pro-Russian Ukrainian rebels and condemning an upsurge in violence that has claimed dozens of lives in Ukraine, council diplomats said.

Pro-Russian rebels have launched an offensive against the strategic port of Mariupol in eastern Ukraine, prompting warnings from the European Union and U.N. Secretary-General Ban Ki-moon.

Fighting has escalated in eastern Ukraine in recent days, and Russian President Vladimir Putin has blamed Kiev. The rebels have ruled out more peace talks. Western powers and Kiev blame Russia and the rebels for the fresh fighting.

Ban's press office issued a statement saying he "strongly condemns today's rocket attack on the city of Mariupol, which reportedly killed dozens of civilians and left over one hundred injured."

It also said Ban condemned the rebel rejection of a ceasefire and "their provocative statements about claiming further territory."

Britain proposed a Security Council statement that would have echoed Ban's rebuke and called for an investigation into the attacks on Mariupol, but Russia rejected it, diplomats said. Council statements need unanimous approval.

"Russia have just blocked a (Security Council) statement by refusing to include condemnation of recent provocative public statements by the separatists," a council diplomat told Reuters on condition of anonymity.

The press office of the Russian U.N. mission said in a statement to Reuters that "consensus has not been reached ... because the delegation of the UK insisted on condemnation of some of the self-defense forces' statements." It added that Western council members have repeatedly refused to condemn aggressive statements and actions by the Kiev government.

A revised version of the draft statement, obtained by Reuters, specifically named Alexander Zakharchenko, head of the rebels' self-proclaimed Donetsk People's Republic in eastern Ukraine, who made clear on Friday the separatists did not want compromise as they were now advancing.

On Saturday Zakharchenko said the Mariupol offensive was "the best possible monument to all our dead" and promised further military advances.

The draft statement "condemned in the strongest terms such irresponsible announcements."

The council has been deadlocked on Ukraine since the start of the conflict a year ago. Russia, which Western council members accuse of sending troops and weapons to bolster the separatists in Ukraine, has veto powers and can block all council action. Moscow denies directing the rebels.