Two officers were killed Tuesday and three hospitalised with toxic burns suffered during a chemical spill at Russia's Plesetsk military launch pad, the defence ministry said.

The accident happened while the soldiers were cleaning a nitrogen gas tank during routine maintenance, Russian news agencies quoted a defence ministry official as saying.

The ministry did not disclose details of the accident, saying only that it was caused "by the officers' failure to follow safety procedures."

The Plesetsk Cosmodrome, located about 800 kilometres (500 miles) north of Moscow, was developed by the Soviet Union as a launch pad for its intercontinental ballistic missiles.

The heavily guarded military installation now serves as Russia's only alternative to the Baikonur space centre Moscow leases from Kazakhstan.