* Paper says secret briefing took place earlier this month
* Israeli officials refuse to comment/confirm report
JERUSALEM, July 29 (Reuters) - President Barack Obama's national security adviser has briefed Israel's Prime Minister Benjamin Netanyahu on a U.S. contingency plan to attack Iran should diplomacy fail to curb its nuclear programme, an Israeli newspaper reported on Sunday.
The Haaretz newspaper said that the U.S. adviser - Thomas Donilon - had described the plan in talks with Netanyahu earlier this month.
A senior Netanyahu aide, Harel Locker, refused to comment on the report when asked about it in an interview with an Israeli radio station. Another Israeli official reached by telephone, said "we do not comment on closed-door diplomatic meetings".
Haaretz said the secret briefing was the most significant effort by high-level U.S. officials who had visited Israel in the past month, including Secretary of State Hillary Clinton, to try to dissuade Israel from launching its own military strike on Iran.
The report coincided with a visit to Israel by Obama's main rival in his reelection bid this November, Republican candidate Mitt Romney, who is due to meet Netanyahu on Sunday.
Quoting a senior U.S. official it said spoke on condition of anonymity, Haaretz said Donilon had told Netanyahu the Pentagon was planning for a possible decision to attack Iran's nuclear sites, and had shown him some of the plans.
In their talks, the same official said Donilon had also detailed the U.S. military's ability to penetrate nuclear facilities buried deep underground, and had said that such contingency plans were being drawn up in case of a possible deadlock in diplomacy with Iran.
The failure of talks between Iran and six world powers to secure a breakthrough in curbing what the West fears is a drive to develop nuclear weapons has raised international concerns that Israel, widely assumed to be the Middle East's only nuclear-armed state, may opt for a go-it-alone military strike.
Israel has warned the West it thinks it is only a matter of time before Iran's nuclear programme achieves a "zone of immunity" in which bombs will not be able to effectively strike uranium enrichment facilities buried deep underground.
Iran says its programme is solely for peaceful purposes.
On a visit to Jerusalem this month, Clinton said Israel and Washington were "on the same page" with respect to Iran, calling Iran's latest proposals to world power talks on the issue "non starters."
"Our own choice is clear, we will use all elements of American power to prevent Iran from obtaining a nuclear weapon," Clinton said.