VIENNA â€"  The head of the U.N. nuclear agency has arrived in Tehran on a key mission that could lead to the resumption of probes on whether Iran has secretly worked on a nuclear weapon.

International Atomic Energy Agency chief Yukiya Amano and his two aides landed early Monday morning in Iran but where quickly whisked away from the Tehran airport.

They are to meet Iran's top nuclear negotiator, Saeed Jalili, as well as the foreign minister and other Iranian officials later in the day.

The visit is focused on getting Iran to agree to terms that will allow IAEA probes of suspect Iranian sites. Tehran denies having worked on atomic weapons.

Western diplomats have expressed skepticism that Iran will honor a deal. But with both Iran and the IAEA reporting progress in a previous round last week, anticipation was high as Amano prepared to board his flight to Tehran.

While expressing some optimism, Amano said he could not predict whether he would clinch a deal that would allow his agency to renew its long-stalled probe.

"Nothing is certain in life, in diplomacy," he told reporters at Vienna's airport. "But there has been good progress.

"I really think this is the right time to reach agreement."

The one-day trip is significant both for what it can achieve in terms of probing Iran's secretive nuclear program and as a mood-setter for talks Wednesday in Baghdad between Iran and the United States, Russia, China, Britain, France and Germany.

The latter six nations are in the forefront of trying to persuade Tehran to curb its nuclear program and ease concerns it wants to use it to make nuclear weapons.

Iran will seek to stay looming U.S. and European Union sanctions on its oil exports at the Baghdad talks.

The six in turn will attempt to get Iran to commit to stop enriching uranium to a level that can be turned quickly into the fissile core of nuclear warheads, while ignoring - for now - its program of lower enrichment, which would take longer to turn toward weapons-making.

Iran insists it is enriching uranium only to produce nuclear fuel. It denies that it worked secretly on developing components of a nuclear arms program, despite what the IAEA describes as credible intelligence and other evidence that it hid work "specific to nuclear weapons."

Amano's visit and the talks in Baghdad are thus separate but indirectly related - a point that Amano touched on as well, saying he hoped they "will give (a) positive good impact (on) each other." His lead partner in the Tehran talks will be Saeed Jalili, Iran's chief nuclear negotiator, who will also lead his country's delegation to Baghdad.

Western diplomats following the IAEA's work are skeptical that Iran would honor the terms of any deal suddenly allowing the IAEA access to sites, people and documents it seeks in its probe, pointing out that Tehran has stonewalled the agency's efforts since 2007. They say Tehran is seeking to make points ahead of the Baghdad talks, where it would refer to any deal with the IAEA as a sign of its good will and demand that the upcoming sanctions on Iranian oil be suspended.

Diplomats told The Associated Press ahead of the Baghdad talks that there is agreement among the six powers not to give in to such demands. G-8 leaders last week set the stage for a united release of world oil reserves to balance any disruption in world markets when those tough new sanctions are imposed. President Barack Obama said world powers "are unified in our approach to Iran."

Still, an Iran-IAEA deal could reopen fissures among the six, with Russia and China - traditional opponents of tough anti-Iran sanctions - demanding that the oil penalties be held up to reward the Islamic Republic's approval of a deal with the IAEA.

Russian Deputy Foreign Minister Sergei Ryabkov said the Baghdad talks should already focus on lifting of sanctions if Iran shows signs of compromise, in comments reported Sunday by the Itar-Tass news agency.

"Russia denies the efficiency of sanctions against Iran; it thinks that the sanctions are driving the problem into an impasse," he was quoted as saying. "Yet bearing in mind the adherence of Western partners to sanctions, I think they should think about the time when the sanctions may be suspended and lifted."

Amano deferred an Iranian invitation to visit Tehran last year, saying he would go only if he was assured of progress in the standoff over the IAEA probe. Pressed at the airport whether he expected to come back with a deal in his pocket, he repeated that "nothing is certain but ... I stay positive."