U.S. President Barack Obama on Tuesday urged all countries, including Israel, to sign the international Nuclear Non-Proliferation Treaty, or NPT.

"Whether we're talking about Israel or any other country, we think that becoming part of the NPT is important," Obama said. "And that, by the way, is not a new position. That's been a consistent position of the United States government, even prior to my administration."

Israel operates a policy of 'nuclear ambiguity,' refusing to confirm or deny having atomic weapons - but has not signed the treaty and is believed by analysts to posses between 200 and 300 nuclear warheads.

Obama, who was hosting an international conference on nuclear security in Washington, was at first reluctant to answer journalists' questions on Israel, whose prime minister, Benjamin Netanyahu had at the last minute canceled plans to attend.

"Initially you were talking about U.S. behavior, and then suddenly we're talking about Israel. Let me talk about the United States," the president said.

"I do think that as part of the NPT, our obligation, as the largest nuclear power in the world, is to take steps to reducing our nuclear stockpile. And that's what the START treaty was about, sending a message that we are going to meet our obligation...as far as Israel goes, I'm not going to comment on their program.

Iran sanctions

Obama also pressed for "bold, swift" sanctions on Iran, which the West suspects of developing a bomb in contravention of the NPT, to which it is signatory.

But acknowledged China has concerns about the economic impact and said negotiations are difficult.

Iran's nuclear standoff with the West cast a shadow over the summit.

A day after discussing Iran with Chinese President Hu Jintao, Obama expressed gratitude that China had agreed to help negotiate a new UN sanctions resolution on Iran but said Beijing still has concerns about the issue.

He said he had argued to Chinese President Hu Jintao in talks on Monday that there must be consequences for Iran's violations of its international obligations.

"The Chinese are obviously concerned about what ramifications this might have on the economy generally," Obama said. "Iran is an oil-producing state."

Obama on March 30 said he hoped to get a new sanctions resolution ready within weeks. He declined to provide a timetable on Tuesday but said he did not want a long, drawn-out process that takes months.

"I think that we have a strong number of countries on the Security Council who believe this is the right thing to do. But I think these negotiations can be difficult and I am going to push as hard as I can," he said.

French President Nicolas Sarkozy said the time has come for countries to act decisively in the face of Iran's continued defiance: "The moment of truth is now."

Iranian President Mahmoud Ahmadinejad, who was not invited to the summit, refused to back down, telling state television Tuesday that Iran's "nuclear rights and path are untouchable" despite US-led pressure.

"I believe that the ballyhoo over the nuclear issue is just an excuse by the US to weaken Iran and get domination over the Middle East," Ahmadinejad said.

Obama downbeat about Mideast peace prospects

Also Tuesday, Obama gave a surprisingly downbeat assessment of the chances for a U.S.-brokered peace settlement in the Middle East, saying that the United States cannot help if Israel and the Palestinians decide they cannot negotiate.

The two sides may say to themselves, "We are not prepared to resolve these issues no matter how much pressure the United States brings to bear," Obama said.

Obama reiterated that peace is a vital goal, but one that may be beyond reach even if we are applying all of our political capital.

Obama was responding to a question about whether the successful negotiation of a new arms control treaty with Russia and the agreements he won at this week's nuclear summit could help him make gains elsewhere. His words are a recognition that although he pledged to work hard for a deal from his first day in office, Obama has gotten little traction in the decades-old conflict.

The United States is pushing for new Israeli-Palestinian talks in which the the Obama administration would be a go-between. Previous talks broke off more than a year ago, and despite shuttle diplomacy and unusual pressure on ally Israel, the Obama administration has been unable to reach even the modest goal of new talks.