Iran's president has insisted enriching uranium to 20 percent "is our right" and not a step towards a bomb, as a US envoy warned the window for dialogue over Tehran's nuclear programme was closing.

The enrichment activity, which world powers are trying to curb in fraught talks with Iran, "is one of our rights in terms of international law", President Mahmoud Ahmadinejad said Wednesday in an interview with the satellite television network France 24.

"There have been lies about our programme... Enriching uranium to 20 percent is not a step towards a bomb," he insisted, speaking Farsi through translators.

Ahmadinejad hinted however that Iran could be open to stopping 20 percent enrichment -- if world powers offer significant concessions.

"If others do not wish for us to fully benefit from this right, they need to explain to us why. And also they have to say what they are willing to give to the Iranian people in exchange."

US ambassador to Israel, Dan Shapiro, warned that the window for dialogue over Iran's nuclear programme was closing, insisting that Washington was under no illusions about Tehran's agenda in the talks.

Speaking at Tel Aviv University a week after world powers held a second round of largely fruitless talks with Iran, the US ambassador said Washington would not continue such dialogue forever.

"We don't intend on continuing talks for talks' sake. The window is closing," he told participants attending an international security conference at the university.

"We have no illusions that Iran may be using this to buy time," he said. "It is up to Iran to follow through. The burden falls on Iran to prove it is serious."

Iran and the P5+1 -- Britain, China, France, Russia and the United States, plus Germany -- achieved little in two days of intense talks in Baghdad last week, except for setting up another meeting in Moscow in June.

Shapiro said there were "significant differences" at the Baghdad talks, but also "a narrow common ground", which the parties would try to build on in the Russian capital.

And he said the threat of military action against Iran's nuclear facilities was still open.

"As we apply all elements of American power to prevent a nuclear Iran, the United States takes no option off the table -- that means a political component, a diplomatic component, an economic component and a military option," he said.

Neither the US nor Israel has ruled out the option of a military strike, but the administration of President Barack Obama has made clear it favours diplomacy over force at this stage.

The UN Security Council has issued six resolutions demanding Iran suspend all uranium enrichment. It has also imposed four sets of sanctions on Iran, which Western powers have hardened with their own harsh economic sanctions.

The West suspects Tehran is secretly developing nuclear weapons capability.

Iran, which insists its nuclear programme is peaceful, enriches uranium to 3.5 percent for its Bushehr atomic energy plant in the south of the country., but insists it needs 20 percent uranium to create medical isotopes in its Tehran research reactor.

"Why should the 20 percent enrichment create doubt? The Western powers have nuclear bombs. Should we trust them? Which is more dangerous, an atomic bomb or the 20 percent (enrichment)?" Ahmadinejad asked.

The 20 percent enrichment issue is at the heart of the talks with the P5+1 that are due to resume June 18-19 in Moscow.

Ahmadinejad said that as much as Iran would like to see the nuclear dispute resolved, "we do not expect to see a miracle" in Moscow.

He complained that Western governments did not protest when Israel threatened Iran, adding: "We are not afraid of their threats. The Iranian people have shown they know how to deal with such situations."