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Now is the time to act to halt the Iranian nuclear program, Prime Minister Benjamin Netanyahu told U.S. House Speaker Nancy Pelosi and other Senators and Congress members over the weekend.

The prime minister's message, which he made in telephone conversations, came a day after evidence of a clandestine Iranian nuclear facility was presented by U.S. President Barack Obama and the leaders of Britain and France at the G-20 economic summit in Pittsburgh.

"If not now, then when?" Netanyahu reportedly asked in the conversations with American officials.

Meanwhile, Foreign Minister Avigdor Lieberman said on Saturday that the newly exposed nuclear facility in Iran was built for military purposes and was proof that the Islamic Republic is seeking atomic weapons.

"I spoke this weekend with experts from the East and West," Lieberman told Israel Radio. "No one has any doubt, according to the technical data that was published, it's a military core. The disagreement has been done away with."

Israeli leaders have repeatedly expressed alarm over Iran's nuclear ambitions and refused to rule out pre-emptive military action to stop Iran developing an atomic weapon.

Tehran insists its nuclear plans are peaceful and has resisted U.S.-led diplomatic pressure to curb its uranium enrichment, which can be used to generate electricity or make bombs.

On Friday, U.S. President Barack Obama demanded that Iran come clean about its nuclear program or risk "sanctions that bite," after the disclosure that a new plant was under construction southwest of Tehran.

Iranian President Mahmoud Ahmadinejad said the facility was legal and open for inspection by the United Nations International Atomic Energy Agency. Iran is set to meet the United States and five other powers in October to discuss its nuclear program.

Asked whether the strong words from Obama and other Western leaders came too late, Lieberman said: "No one wanted to admit the truth. Everyone tried, like always, to placate this mad regime."

"I hope that after this latest revelation, at the discussion to open on October 1... the message passed from the start will be unequivocal," he said.

Israel considers Iran a strategic threat due to its nuclear program, missile development and repeated references by Ahmadinejad to Israel's destruction.

Lieberman said he met with Arab foreign ministers while at the United Nations last week and said they expressed their alarm over Iran's nuclear program to him.

"Nobody is worried about the Palestinian problem, everybody in the Muslim and Arab world, and first and foremost in the Gulf states, are worried about the Iranian problem," the foreign minister said.

Obama issues ultimatum to Iran over nukes

Not letting up on Iran, Obama said Saturday that the Islamic republic must take action to demonstrate its peaceful intentions after the discovery of the covert nuclear facility, or it will be held accountable by the world's nations.

In his weekly radio and Internet address, Obama said that evidence showing Iran building a new underground plant to enrich uranium that could be used for an atomic bomb continues a disturbing pattern of Iranian evasion that jeopardizes global nonproliferation.

He urged Tehran once again to open the site to international inspectors, or face consequences. The chief option is tougher economic sanctions, but on Friday Obama and administration officials did not rule out military action.

"My offer of a serious, meaningful dialogue to resolve this issue remains open," Obama said. "But Iran must now cooperate fully with the International Atomic Energy Agency, and take action to demonstrate its peaceful intentions."

"Iran's leaders must now choose - they can live up to their responsibilities and achieve integration with the community of nations. Or they will face increased pressure and isolation, and deny opportunity to their own people," he said.

Evidence of the clandestine facility was unveiled Friday by Obama and the leaders of Britain and France at the G-20 economic summit in Pittsburgh, where it overshadowed developments on regulating financial markets and reducing fossil fuel subsidies.

Soon after, Russian President Dmitry Medvedev, at his own news conference, urged Iran to cooperate, as did Chinese Vice Foreign Minister He Yafei. He, however, did not endorse sanctions against the country.

"On this, the international community is more united than ever before ... that Iran must fulfill its responsibilities," Obama said.

Iran, so far, hasn't budged.

At a news conference in New York, Ahmadinejad said his country had done nothing wrong and Obama would regret his actions.

Ahmadinejad said the plant - which Iranian officials say was reported to nuclear authorities as required - wouldn't be operational for 18 months. But he sidestepped a question about whether Iran had sufficient uranium to manufacture a nuclear weapon.

The head of Iran's nuclear program suggested that United Nations inspectors may be allowed to visit the site. Ali Akbar Salehi called the incomplete facility a semi-industrial plant for enriching nuclear fuel, but he gave no other details, according to the state news agency IRNA.

Obama, and other world leaders, will be looking to see where Iran stands next week during a meeting of major nations on the nuclear issue.

Obama said Saturday the negotiations have taken on added urgency.