(Reuters) - Israel has asked the United States for advanced bunker-buster bombs and refueling planes that could improve its ability to attack Iran's underground nuclear sites, an Israeli official said Thursday.

Such a request was made around the time of Israeli Prime Minister Benjamin Netanyahu's visit to Washington this week, the official told Reuters, confirming media reports.

But the official, who spoke on condition of anonymity given the sensitivity of the issue, played down as unrealistic Israeli reports that the United States would condition supplying the hardware on Israel promising not to attack Iran this year.

White House spokesman Jay Carney, asked whether the Israelis had made such a request to U.S. officials during the visit, said there was no such agreement proposed or reached in President Barack Obama's meetings with Netanyahu or his aides.

But when asked if the matter was raised with Defense Secretary Leon Panetta or other U.S. officials, Carney told reporters he had no information on that. I would refer you to other officials, he said.

A U.S. official, speaking on condition of anonymity, confirmed that military capabilities came up in discussions between Netanyahu and Panetta but did not elaborate. No deals were struck during those talks, the official added.

Netanyahu made clear to Obama at a White House meeting on Monday that Israel had not yet decided on military action against Iran, the White House has said.

Netanyahu has hinted that Israel could resort to force should Tehran - which denies suspicions that it is covertly trying to develop atomic bombs - continue to defy big powers' diplomatic pressure to curb its nuclear program.

The risk of an Israeli-Iranian war troubles Obama, who is up for re-election in November and has cautioned against sparking greater Mideast turmoil, though he has also asserted that military action remains an option if sanctions fail. A Gulf conflict could send oil prices soaring.

A front-page article in the Israeli newspaper Ma'ariv on Thursday said Obama had told Netanyahu Washington would supply Israel with upgraded military equipment in return for assurances there would be no attack on Iran in 2012.

Israel is widely assumed to have the Middle East's only nuclear arsenal but its conventional firepower may not be enough to deliver lasting damage to Iran's distant, dispersed and well-fortified facilities, many experts say.

Israel has limited stocks of older, smaller bunker-busting bombs and a small fleet of refueling planes, all supplied by Washington.

Western powers suspect Iran's uranium enrichment program is aimed at stockpiling fissile material for nuclear weapons. Iran says it is strictly for civilian energy uses.

(Writing by Maayan Lubell, additional reporting by Matt Spetalnick, Alister Bull and Phil Stewart in Washington; editing by Mark Heinrich and Todd Eastham)