Biden's remarks suggested a tougher U.S. stance against Iran's nuclear ambitions. Nonetheless, administration officials insisted his televised remarks reflected the U.S. view that Israel has a right to defend itself and make its own decisions on national security.

In an interview on ABC's "This Week," Biden also said that the U.S. offer to negotiate with Tehran on its nuclear program still stands.

Some thought the administration's approach might change in light of the Iranian government's crackdown on protesters after the June 12 presidential election. Opponents of the ruling authorities said the vote was rigged against them.

"If the Iranians respond to the offer of engagement, we will engage," Biden said.

It was after meeting Israeli Prime Minister Benjamin Netanyahu at the White House on May 18 that President Obama said it should be clear by year's end whether Iran was open to direct negotiations.

Obama said last Thursday that persuading Iran to forego nuclear weapons has been made more difficult by the crackdown after the disputed re-election of President Mahmoud Ahmadinejad.

Biden was asked whether Netanyahu was taking the right approach by indicating that Israel would take matters into its own hands if Iran did not show a willingness to negotiate by the end of the year.

"Look, 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," Biden replied. He added that this was the case, "whether we agree or not" with the Israeli view.

Biden was then asked more pointedly whether the United States would stand in the way if the Israelis, viewing the prospect of an Iranian nuclear bomb as a threat to the existence of the Jewish state, decided to launch a military attack against Iranian nuclear facilities.

"Look, we cannot dictate to another sovereign nation what they can and cannot do," he said.

Pressed further on this point with a reminder that the United States could impede an Israeli strike on Iran by prohibiting it from using Iraqi air space, Biden said he was "not going to speculate" beyond saying that Israel, like the U.S., has a right to "determine what is in its interests."

Navy Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said Sunday that he has been "for some time concerned about any strike on Iran." He also said military action should not be ruled out and that a nuclear-armed Iran is a highly troubling prospect.

In Jerusalem, the Israeli government had no comment on Biden's remarks.

White House spokesman Tommy Vietor said Biden was not signaling any change of approach on Iran or Israel.

"The vice president refused to engage hypotheticals, and he made clear that our policy has not changed," Vietor said. "Our friends and allies, including Israel, know that the president believes that now is the time to explore direct diplomatic options."

Mullen, who as Joint Chiefs chairman is the top military adviser to Obama and Gates, said he worries about unpredictable consequences of an attack on Iran.

"I worry about it being very destabilizing not just in and of itself but the unintended consequences of a strike like that," he told CBS's "Face the Nation." "At the same time, I'm one that thinks Iran should not have nuclear weapons. I think that's very destabilizing."

Mullen said he worries that, in the event Iran were to obtain a nuclear weapon, other countries in the Middle East would feel compelled to follow suit. That would open a door to a proliferation of nuclear technology that would be destabilizing, Mullen said, adding that this is a subject he discusses regularly with his Israeli counterpart.

The prospect of a regional nuclear arms race was raised by Obama in an AP interview Thursday.

"The biggest concern is not simply that Iran can threaten us or our allies like Israel or its neighbors in the region," Obama said. "A very real concern is, is that Iran possessing a nuclear weapon triggers an arms race in the region and suddenly countries like Egypt and Saudi Arabia and Turkey all feel obliged to get nuclear weapons. And if you've got the most volatile region in the world and everybody armed to the teeth with nuclear weapons, you've got a recipe for potential disaster."

Most experts believe that wiping out the Iranian nuclear program is beyond the ability of Israel's military. In 1981 the Israeli air force destroyed Iraq's nuclear reactor in a lightning strike. But Iran's facilities are scattered around the country, some of them underground. *