You are an expert at summarizing long articles. Proceed to summarize the following text: 
russian planes on sunday bombed rebel armored units and bridges near grozny , capital of the breakaway caucasian republic of chechnya , hours after a midnight ultimatum expired for separatists to lay down their arms , russian officials were quoted as saying by the itar-tass agency .
russia bombs outskirts of grozny yeltsin urges peace