Barack Obama vowed Monday that Iran would not develop nuclear weapons so long as he is president and pledged full support to Israel, as he faced criticism from challenger Mitt Romney.

"As long as I am president of the United States, Iran will not get a nuclear weapon," Obama said in the third and final debate before November 6 presidential elections.

"Israel is a true friend. It is our greatest ally in the region. And if Israel is attacked, America will stand with Israel. I've made that clear throughout my presidency," he said.

Reiterating comments from his aides, Obama denied a weekend report in The New York Times that the United States and Iran -- which have no diplomatic relations -- were prepared for one-on-one talks after the election.

"Those are reports in the newspaper. They are not true," Obama said.

But Obama, accusing Romney of waffling on his core positions, told him he was "pleased that you now are endorsing our policy of applying diplomatic pressure and potentially having bilateral discussions with the Iranians" to end their nuclear program.

Obama, who offered talks with US adversaries at the start of his term, said that Iran was increasingly isolated due to international sanctions.

Romney accused Obama of failing to stop progress in Tehran's nuclear program, which Israel and some Western officials say is aimed at developing a nuclear bomb but the clerical regime insists is for peaceful purposes.

"It is absolutely the right thing to do to have crippling sanctions. I'd have put them in place earlier but it is good that we have them," Romney said. "Something I'd add today, I would tighten those sanctions."

"I see Iran four years closer to a bomb," Romney said, as he also criticized Obama's record in other parts of the Middle East including violence-torn Syria.

"I don't see our influence growing around the world; I see our influence receding," Romney said.