President Barack Obama and Vice President Joe Biden, in separate interviews this weekend, said that the accelerating crackdown on opposition leaders in Iran in recent days would not deter them from seeking to engage the country's top leadership in direct negotiations.

In an interview with The New York Times, a day before his scheduled departure for Moscow on Sunday, Obama acknowledged the arrests and intimidation of Iran's opposition leaders, but insisted, as he has throughout the Iranian crisis, that the repression of those leaders would not close the door on negotiations with the Iranian regime.

"We've got some fixed national security interests in Iran not developing nuclear weapons, in not exporting terrorism, and we have offered a pathway for Iran to rejoining the international community," Obama said.

Biden echoed those themes in an interview conducted in Iraq and broadcast Sunday on ABC's "This Week." But in a rare foray into one of the most sensitive issues in the Middle East, the vice president argued that the United States "cannot dictate" to Israel whether to strike the facilities at the heart of Iran's nuclear program if Israeli leaders determine "that they're existentially threatened" by the prospect that Iran would gain nuclear-weapons capability.

The emphasis was different in a separate appearance by the chairman of the Joint Chiefs of Staff, Adm. Mike Mullen, who warned that an Israeli strike on Iran "could be very destabilizing." Asked to choose between Israeli military action and permitting Iran to gain nuclear-weapons capability, he said both would be "really, really bad outcomes."

Prior to Iran's disputed election on June 12, the president's top aides say, they received back-channel indications from Iran - from emissaries who claimed to represent the supreme leader, Ayatollah Ali Khamenei - that the country would be ready to respond to Obama's overtures this summer.

But the crackdown and the divisions among senior clerics about the legitimacy of the election and Khamenei's own credibility have changed the political dynamics in both Washington and Tehran. Senior administration officials said they have heard nothing from Iran's leaders.

The Obama administration, meanwhile, has been preparing for two opposite possibilities: one in which the Iranian leadership seeks to regain a measure of legitimacy by taking up Obama's offer to talk - a situation that could put Washington in the uncomfortable position of giving credibility to a government whose actions Obama has deplored - or one in which Iran rejects negotiations.

Obama told Prime Minister Benjamin Netanyahu of Israel during a meeting at the White House in May that if there were no progress on the Iranian nuclear issue by year's end, the administration would turn to other steps, including sanctions. Obama seemed to hint at an even shorter schedule during the interview on Saturday.

"We will have to assess in coming weeks and months the degree to which they are willing to walk through that door," he said.

Obama declined to talk about the preparations for a tougher line. But as he prepared to leave on Sunday for Moscow, he said that he believes that the United States now has more leverage to pressure Iran because he had succeeded in getting "countries like Russia and China to take these issues seriously," noting that both countries had approved escalating sanctions on North Korea.

By drawing the parallel, he seemed to suggest Russia and China could prove willing to do the same with Iran. But the calculus will be more difficult: China's ravenous energy needs make it a large customer of Iranian oil, and Russia sees Iran as a huge market, including for nuclear reactors.

In his interview, Biden ventured into what is usually forbidden territory by openly discussing the possibility that Israel may decide it cannot wait to see if Obama's diplomatic overtures work.

"Israel can determine for itself - it's a sovereign nation - what's in their interest and what they decide to do relative to Iran and anyone else," he said. But he quickly added that the United States would not let any other nation determine its approach to national security either, including the wisdom of engagement. "If the Iranians respond to the offer of engagement, we will engage," he said. "The ball's in their court."

Israeli officials have been deeply uncomfortable with Obama's engagement offer, arguing that while the two countries circle each other, Iran is adding new centrifuges to its facility at Natanz, where it can enrich uranium. The last report of the International Atomic Energy Agency indicated roughly 7,000 centrifuges are operating, but turning out fuel that, without further enrichment, is suitable only for nuclear power.

Last spring, when President George W. Bush was still in office, Israeli officials approached the White House seeking bunker-busting bombs, refueling capability for its military aircraft, and overflight rights over Iraq necessary to strike Natanz. Bush deflected those requests.

U.S. officials have said it is unlikely that Netanyahu would ask Obama for similar help. But that does not mean Israel cannot look elsewhere to develop and obtain the capability it requires.

In comments on CBS' "Face the Nation," Mullen, the Joint Chiefs chairman, seemed to underscore the Pentagon's concern that an Israeli strike could trigger broader conflict, and might simply drive the Iranian nuclear efforts deeper underground. He said any strike on Iran could be "very destabilizing - not just in and of itself but the unintended consequences of a strike like that."

The implication was that following an attack on its nuclear sites, counterstrikes could be expected by Iran or its proxies, aimed at the United States, its troops in the region, or its allies.

In the interview on Saturday, Obama seemed to acknowledge that the administration was still struggling for the right strategy to stop nations from obtaining nuclear weapons capability, after so many previous mixtures of inducements and threats have failed.

"You know, I don't think any administration over the last decade has had the perfect recipe for discouraging North Korea or Iran from developing nuclear weapons," he said, in what was clearly intended as droll understatement. "We know that it is going to be a tough slog. But what I'm confident in is that we are in a much stronger position" to press for the systems to be abandoned, he said, now that the United States is committed to a reducing its own stockpiles.