President Obama and Vice President Joseph R. Biden Jr., 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, Mr. Obama said he had "grave concern" about the arrests and intimidation of Iran's opposition leaders, but insisted, as he has throughout the Iranian crisis, that the repression would not close the door on negotiations with the Iranian government.

"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," Mr. Obama said.

Mr. Biden echoed the same themes in an interview conducted in Iraq and broadcast Sunday on the ABC News program "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" Israel's decisions about whether to strike the plants at the heart of Iran's nuclear program. He said only Israelis could 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 any military strike on Iran "could be very destabilizing." Asked to choose between military action and permitting Iran to gain nuclear weapons capability, he said both would be "really, really bad outcomes."

Before 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 respond to Mr. Obama's overtures this summer. But the crackdown and the divisions among senior clerics about the legitimacy of the election and Ayatollah Khamenei's credibility have changed the political dynamics. Senior administration officials said they have heard nothing from Iran's leaders.

The 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 Mr. Obama's offer to talk - a situation that could put Washington in the uncomfortable position of giving credibility to a government whose actions Mr. Obama has deplored - or one in which Iran rejects negotiations. Mr. Obama told Prime Minister Benjamin Netanyahu of Israel in May that if there were no progress on the Iranian nuclear issue by the year's end, the administration would turn to other steps, including sanctions. Mr. Obama hinted 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.

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

In his interview, Mr. Biden ventured into what is usually forbidden territory by discussing the possibility that Israel may decide it cannot wait to see if Mr. 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 added that the United States would not let any other nation determine its approach to national security, including the wisdom of engagement. "If the Iranians respond to the offer of engagement, we will engage," he said.

Israeli officials have been deeply uncomfortable with Mr. Obama's engagement offer, arguing that Iran is still adding centrifuges to its plant at Natanz, where it can enrich uranium. The last report of the International Atomic Energy Agency indicated roughly 7,000 centrifuges are now enriching uranium into fuel, but without further enrichment it is suitable only for nuclear power.

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

American officials have said it is unlikely that Mr. Netanyahu would ask Mr. Obama for similar help. But that does not mean Israel cannot look elsewhere to develop and obtain that ability.

In comments on the CBS News program "Face the Nation," Admiral Mullen seemed to underscore the Pentagon's concern that an Israeli strike could start a 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 plants, counterstrikes could be expected by Iran or its proxies, aimed at the United States, its troops in the region or its allies.

In the Saturday interview, Mr. Obama seemed to acknowledge that the administration was still struggling for the right strategy to stop nations from obtaining nuclear weapons capacity, after so many mixtures of inducements and threats had 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."

Reporter Released in Iran

A freelance reporter for The Washington Times detained in Iran almost three weeks ago was released Sunday, according to news reports.

The reporter, Iason Athanasiadis, who has British and Greek citizenship, had been arrested on June 17 and accused of "illegal activities" during the protests that followed the June 12 election.