By Matt Robinson

PRISTINA, Serbia, April 18 (Reuters) - Romanian U.N. police who shot dead two ethnic Albanian protesters in Kosovo in February fired rubber bullets with a use-by date of 1994, which had "probably hardened" with age, police said on Wednesday.

A preliminary U.N. report released this week blamed a Romanian contingent of the 1,800-strong U.N. police force for the deaths, caused by head wounds from Italian-made RB1-rubber bullets fired at close range.

The bullets, brought into Kosovo by the Romanian contingent, were manufactured in 1991 and had a shelf life of three years, Commissioner Richard Monk, U.N. police chief in the breakaway Serbian province, told a news conference.

"Probably the rubber hardened over time, which given the muzzle velocity would make them even more risky to use at close range," he said.

Monk said the RB1 bullets, used only by the Romanians during the deadly Feb. 10 protest, were smaller and faster than rubber baton rounds. He had directed that such ammunition in Kosovo be either sent home or destroyed.

The deaths shocked Kosovo and fuelled Western fears of widespread unrest if the U.N. Security Council does not decide soon on a Western-backed plan for independence.

An interim report by U.N. Special Prosecutor Robert Dean, released on Tuesday, found that neither of the victims posed a threat when shot and that there was a "reasonable suspicion that such shooting was criminal".

One of the victims was shot behind the ear as he ran through the lobby of a hotel, away from the scene of the protest.

The investigation had not identified which of the officers fired the fatal shots, so charges could not be brought. Bucharest replaced the Romanian contingent last month, despite a U.N. request for 11 officers to stay on pending the inquiry.

Monk said the United Nations in New York was investigating how it was possible that out-of-date ammunition could be used, but added: "It wouldn't be within the U.N. capacity to issue regulations about the age of bullets. We would expect that units coming here would adhere to the manufacturer's rules."

Kosovo's interior minister and the British U.N. police chief both resigned over the violence, the worst since 2004.

The province of 2 million people -- 90 percent ethnic Albanian -- has been run by the United Nations since 1999, when NATO bombs drove out Serb forces accused of atrocities in a two-year war with separatist guerrillas.

Around 3,000 Albanians took part in the February protest, demanding full and immediate independence and rejecting a U.N. plan for statehood supervised by the European Union.

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