DigitalGlobe satellite file image, received courtesy of the Institute for Science and International Security, showing a view of facilities within Parchin, Iran.
Updated at 8:25 a.m. ET: TEHRAN -- Iran said on Tuesday it would let U.N. nuclear investigators visit a military complex where they had been refused access to check intelligence suggesting Tehran has pursued explosives research relevant to nuclear weapons.
Western diplomats dismissed Iran's statement as a time-buying gambit -- rather than a genuine shift toward nuclear transparency -- with Israel talking increasingly stridently of last-resort military action against its arch-enemy.
Diplomats cited a proviso in the statement saying that access to Parchin still hinged on a broader agreement on how to settle outstanding issues which the two sides have long been unable to reach -- an impasse that has put the West and Tehran on a slippery slope toward confrontation.
The West has sharpened sanctions against Iran to block its oil exports, a defiant Tehran has threatened to shut Gulf oil shipping lanes in reprisal while Israel has signaled it is losing patience with efforts to isolate the Islamic Republic.
Meanwhile, European Union foreign policy chief Catherine Ashton accepted an offer to meet Iranian nuclear negotiators to discuss Tehran's nuclear program.
Ashton represents six powers -- the United States, Russia, China, France, Britain and Germany -- in dealings with Iran, and her offer of talks came after weeks of consultations with them.
In a meeting between the two world leaders, Israel's Prime Minister Benjamin Netanyahu did not back down from possible military action against Iran, while President Barack Obama said there is a still a window during which sanctions could work. NBC's Andrea Mitchell reports.
An International Atomic Energy Agency report last year said that Iran had built a large containment chamber at Parchin, southeast of Tehran, to conduct explosives tests that are "strong indicators" of efforts to develop an atom bomb.
The IAEA requested access to Parchin during high-level talks in Tehran in February, but the Iranian side did not grant it.
"...Parchin is a military site and accessing it is a time-consuming process, therefore visits cannot be allowed frequently ... We will allow the IAEA to visit it one more time," Iran's diplomatic mission in Vienna said in a statement, according to ISNA.
It did not give a date for such a visit. Iranian diplomats and IAEA officials were not immediately available for comment.
Western suspicions about activities at Parchin date back to at least 2004, when a prominent nuclear expert assessed that satellite images showed it might be a site for research and experiments applicable to nuclear weapons.
IAEA inspectors did in fact visit Parchin in 2005 but did not see the place where the U.N. watchdog now believes the explosives chamber was built.
The IAEA named Parchin in a detailed report in November that lent independent weight to Western fears that Iran is working to develop an atomic bomb, an allegation Iranian officials deny.
President Obama met with Israeli Prime Minister Benjamin Netanyahu at the Oval Office and discussed the crisis over potential Iranian nuclear weapons. Msnbc's Thomas Roberts has the latest.
Agency chief Yukiya Amano said on Monday Iran has tripled its monthly production of higher-grade enriched uranium and the U.N. nuclear watchdog had "serious concerns" about possible military dimensions to Tehran's atomic activities.
The different announcements and diplomatic moves came as President Barack Obama and Israeli Prime Minister Benjamin Netanyahu ended two days of public posturing on Iran without appearing to make much progress. Obama wants Israel to refrain from attacking Iran now, and Netanyahu pointedly refused to make that promise.
At the start of their White House meeting Monday morning, Obama and Netanyahu tried to present a united front on the threat emanating from Iran. The U.S. leader reaffirmed that he would resort to military force, if necessary, to keep Iran from getting a bomb and said the U.S. "always has Israel's back where Israel's security is concerned."
But the two men were unable to plaster over differences on how urgently military force might be needed.
President Barack Obama said he's not bluffing about using military action if needed to stop Iran from getting a nuclear weapon. NBC's Andrea Mitchell reports.
Obama never made a direct, public appeal to Israel's visiting leader, but his message was clear.
"We do believe that there is still a window that allows for a diplomatic resolution to this issue," Obama said before the two-hour meeting with Netanyahu in the Oval Office.
Netanyahu thanked Obama for acknowledging that Israel will make its own choices but, for the second time in two days, ignored the American president's appeal to give diplomacy and sanctions time to percolate. Instead Netanyahu emphasized Israel's right to defend itself militarily and suggested he would not be swayed from going it alone if he thought Israel had to move faster to protect itself.
The very purpose of the Jewish state, he told Obama, is "to restore to the Jewish people control over our destiny," he said.
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Reuters and The Associated Press contributed to this report.