By DAVID E.SANGER


President Obama and Vice President Joseph R.Biden Jr., in separate interviews recently, said that the accelerating crackdown on opposition leaders in Iran would not deter them from seeking to engage the country’s top leadership in direct negotiations.

In an interview with The New York Times, 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 July 5. 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.

But the chairman of the Joint Chiefs of Staff, Admiral Mike Mullen, warned that any military strike on Iran “could be very destabilizing.” Asked to choose between military action and permitting Iran to gain nuclear weapons , 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.

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. “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 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.