A unilateral attack by Israel against Iran to thwart the Islamic republic's nuclear ambitions would be an "absolute catastrophe," AFP quoted French President Nicolas Sarkozy as warning on Thursday.

Sarkozy was speaking after a summit of the Group of Eight and other leaders in Italy at which they agreed on the need to pursue a negotiated deal with Tehran to halt its nuclear program.

Earlier this week, U.S. President Barack Obama rebuffed suggestions that Washington had given Israel a green light to attack Iran's nuclear facilities. Israel believes that a nuclear Iran would pose an existential threat.

The French president said Wednesday that major powers in the G8 would give negotiations with Iran a chance until September.

After Sarkozy's statement Wednesday, a senior adviser to Iran's top authority said in remarks published on Thursday that the Islamic Republic would not back down "even one step" over its nuclear work, making clear Tehran's continued defiance toward the West.

Ali Akbar Velayati, Supreme Leader Ayatollah Ali Khamenei's top adviser on international affairs, said Western countries did not want the Islamic state to have peaceful nuclear activities, state broadcaster IRIB said on its website.

Speaking after talks with fellow G8 leaders, Sarkozy said they would review the situation at a G20 meeting of developed and developing countries in Pittsburgh on Sept. 24 and 25.

"If there is no progress by then we will have to take decisions," said Sarkozy, indicating that tougher sanctions might be imposed if Tehran continued to resist negotiations.

Western countries believe Iran is trying to build an atomic bomb. Tehran insists it wants to master nuclear technology to generate electricity, and has rejected all overtures for talks.

U.S. and Canadian officials, meanwhile, said the world's main industrialized nations were growing increasingly impatient.

"All G8 nations are united. There is a strong consensus at the table that unless things change soon, there will be further action," said Canadian spokesman Dimitri Soudas.

White House official: G8 talks reflect 'collective impatience' with Iran

A White House deputy national security adviser, Mike Froman, told reporters the G8 discussions had reflected "a collective impatience with Iran."

However, Sarkozy made clear Wednesday Russia was still dragging its feet over the issue and had pushed for more time before considering a fresh round of sanctions.

"We made an effort to agree not to strengthen sanctions straightaway in order to bring everyone on board. The more reserved amongst us agreed that Pittsburgh was the time for decisions," said Sarkozy.

In a separate statement, the G8 said it was committed to finding a diplomatic solution to Iran's nuclear program.

"We sincerely hope that Iran will seize this opportunity to give diplomacy a chance," the statement said.

The G8 statement came after Prime Minister Benjamin Netanyahu's top national security adviser said Wednesday that the United States was distancing itself from Israel's position on Iran.

The statement also deplored the violence in Iran following last month's disputed presidential election and said arrests of journalists and foreigners there were "unacceptable".

Moreover, it repeated criticism of Iranian President Mahmoud Ahmadinejad for denying the Holocaust.

Turning to the Middle East, the G8 reiterated calls for a swift resumption of negotiations between Israel and the Palestinians over the creation of two separate states.

"We also call for the immediate opening of crossings for the flow of humanitarian aid, commercial goods and persons to and from Gaza, in a manner that respects Israel's security," the statement said.