WRAPUP 6-Obama seeks to calm 'drumbeat of war' over Iran
* Obama says will not countenance Iran having nuclear weapons
* EU's Ashton says talks in sight; venue, date to be decided
* Iran's Jalili says to present "new initiatives"
* Year-long diplomatic vacuum has stoked fears of war
* Iran says to let UN nuclear inspectors visit military site (retops with Obama news conference)
BRUSSELS, March 6 (Reuters) - President Barack Obama said on Tuesday the United States 'will not countenance' Iran developing a nuclear weapon but pledged to take a sober approach to dealing with Tehran's nuclear programme.
Obama said the announcement of six-power talks with Iran offered a diplomatic opportunity to defuse the crisis.
Amid mounting speculation that Iran's nuclear sites could be attacked in coming months, the president said that American politicians "beating the drums of war" had a responsibility to explain the costs and benefits of military action.
Earlier, U.S. Defense Secretary Leon Panetta said the United States would take military action to prevent Iran from acquiring a nuclear weapon if diplomacy failed.
"Military action is the last alternative when all else fails," he told the annual policy conference of the biggest U.S. pro-Israel lobbying group. "But make no mistake, when all else fails, we will act."
Six world powers have accepted an Iranian offer for talks on its disputed nuclear programme, the European Union's top diplomat said, after a year's standstill that has increased fears of a slide into a new Middle East war.
The announcement by EU foreign policy chief Catherine Ashton came shortly after Russia called for a resumption of face-to-face dialogue as soon as possible, saying an Iranian letter last month showed it was now ready for serious negotiations.
With Israel speaking increasingly loudly of resorting to military action, the talks could provide some respite in a crisis which has driven up oil prices and threatened to suck the United States into its third major war in a decade.
Iran's nuclear negotiator, Saeed Jalili, wrote to Ashton in February saying Tehran wanted to reopen negotiations and offering to bring unspecified "new initiatives" to the table.
"Today I have replied to Dr. Jalili's letter of Feb. 14," Ashton, speaking on behalf of the six powers after weeks of consultations with them, said in a statement. "I have offered to resume talks with Iran on the nuclear issue."
Ashton, who represents the United States, Russia, China, France, Britain and Germany in dealings with Iran, said the date and venue for the talks would now have to be agreed.
A senior EU official said these talks were not expected before the Iranian new year in two weeks, though there would be a series of preparatory meetings possibly in the coming days.
"Our overall goal remains a comprehensive negotiated, long-term solution which restores international confidence in the exclusively peaceful nature of Iran's nuclear programme, while respecting Iran's right to the peaceful use of nuclear energy," Ashton said in her reply to Jalili.
Western states are likely to tread cautiously, mindful of past accusations that Iran's willingness to talk has been a stalling tactic to blunt pressure and not a route to agreement.
The Islamic Republic's latest approach to the six powers comes at a time when it is suffering unprecedented economic pain from expanding oil and financial sanctions.
The resumption of talks nonetheless could slow a drift towards military strikes to knock out Iran's uranium enrichment programme, which the West fears is geared to producing atomic bomb fuel and Tehran says is for electricity only.
Israel, which says its existence could be threatened if Iran is allowed to develop nuclear weapons, is losing confidence in Western efforts to rein in the Islamic Republic with sanctions and diplomatic pressure.
Israeli Prime Minister Benjamin Netanyahu assured U.S. President Barack Obama on Monday that the Jewish state has made no decision on attacking Iranian nuclear sites, sources close to talks in Washington said. He, however, gave no sign of backing away from the option of military strikes.
But the new prospect of diplomacy contributed to a fall in oil prices on Tuesday, with Brent crude down $1.70 to $122.10 by 1458 GMT.
The senior EU official said there was some reason to believe talks with Iran might be productive.
"The first is that there is clear written commitment by Iran to be willing to address the nuclear issue in talks," the official said. "Second is the unity of the international community ... Third is certainly sanctions."
Russia, which built Iran's first nuclear power plant and has far warmer relations with Tehran than Western nations do, has often stressed the need for talks and said coercive pressure on Tehran is counterproductive.
Russian Deputy Foreign Minister Sergei Ryabkov said last month that global powers must work harder to seek agreement with Iran, warning that Tehran's appetite for concessions was waning as it moves closer to being able to build atomic weapons.
Ryabkov said he hoped fresh talks with Iran would address a proposal by president-elect Vladimir Putin for global powers to formally recognise Iran's right to enrich uranium, Tehran to submit its programme to full IAEA supervision, and international sanctions to be lifted.
UN VISIT TO IRANIAN MILITARY SITE?
Iran said on Tuesday it would let U.N. nuclear inspectors visit a military site where they have been repeatedly refused access to check intelligence suggesting explosives tests relevant to atom bombs has been conducted there.
Diplomats, however, cited a proviso in the Iranian statement saying that access to the Parchin site still hinged on a broader agreement on how to settle outstanding issues which the two sides have been unable to reach for five years.
An International Atomic Energy Agency report in November said that Iran had built a large containment chamber at Parchin, southeast of Tehran, to conduct high-explosives experiments that are "strong indicators" of an effort to design atomic bombs.
Years of tortuous negotiations have often come unstuck over procedural obstacles imposed by Iran since the IAEA first began seeking unfettered access in the country almost a decade ago to check indications of illicit military nuclear activity.
Israel has mooted pre-emptive bombings against Iran, a hawkish approach that Obama - wary of the risk of igniting a new Middle East war and a global surge in oil prices as he seeks re-election in November - has tried to restrain to give time for harsher sanctions and diplomatic pressure to bear fruit.
Israel insists that military action against Iran would be warranted to prevent it from attaining the capability of making nuclear weapons, as opposed to when it actually builds a device. Washington has not embraced that idea.
"The pressure (on Iran) is growing but time is growing short," Netanyahu was quoted by aides as telling Obama.
Later, addressing the influential pro-Israel lobby AIPAC, Netanyahu said: "None of us can afford to wait much longer. As prime minister of Israel, I will never let my people live in the shadow of annihilation."
U.S. officials say that while Iran may be manoeuvring to keep its options open, there is no clear intelligence that it has made a final decision to "break out" with a nuclear warhead. (Additional reporting by Parisa Hafezi in Tehran, Fredrik Dahl in Vienna, Steve Gutterman in Moscow and Jim Wolf in Washington; Writing by Mark Heinrich; Editing by Robert Woodward)