Iran on Sunday hanged a man said to be affiliated to an exiled opposition group, state media reported, despite international pressure on the Islamic republic to halt the execution.

According to the official IRNA news agency, Gholamreza Khosravi Savadjani was convicted of "waging war against God" (moharebeh) by helping the People's Mujahedeen Organisation of Iran (PMOI).

An Israeli defense official said the Jewish state has not been able to stop the build up of Hizbullah's rocket arsenal.

Head of political-military affairs at Defense Ministry, Maj.-Gen Amos Gilad, told a security conference in Tel Aviv that Iran has overseen the construction of Hizbullah's arsenal of 100,000 rockets.

Iran on Tuesday defended Syria's June presidential election, saying the vote could end the three-year-old civil war ravaging its close ally.

The response came as President Bashar Assad registered to stand in the June 3 vote, which is widely expected to return him to office despite the grinding conflict.

Foreign Minister Mohammad Javad Zarif said on Sunday talks with world powers this week are unlikely to result in a comprehensive accord on Iran's controversial nuclear drive.

"This round of talks will be more serious than previous ones," Zarif said in remarks reported by the official IRNA news agency.

An Iranian negotiator said nuclear talks with world powers this week would focus on advanced centrifuges and the unfinished Arak heavy water reactor, the official IRNA news agency reported Sunday.

Iran is due to resume talks Tuesday in Vienna with the P5+1 -- Britain, France, the United States, Russia and China plus Germany -- aimed at reaching a comprehensive accord on its controversial nuclear program following November's landmark interim deal.

The heaviest snowstorm in five decades has blanketed provinces in northern Iran, cutting power supplies and trapping villagers, Iranian media reported Monday.

The storm is "unprecedented for the past 50 years, with two meters (almost seven feet) of snow falling since Friday," a Mazandaran provincial official said, quoted by the media.

A group of former global leaders known as the Elders, tasked with encouraging dialogue between Iran and the international community, is due to arrive in Tehran Sunday, their website said.

The group, formed in 2007, is made up of twelve global leaders who try "to promote peace, justice and human rights," according to their website.

Iranian nuclear chief Ali Akbar Salehi said the first phase of talks with the U.N. nuclear watchdog will be finished by early February, English-language Press TV website reported Friday.

Earlier on Friday, Iran and world powers agreed on how to implement a landmark November deal on containing Tehran's nuclear program, but it must still be approved by each country before it can take effect.

The White House insisted Friday that a U.S. man reportedly working for the CIA when he went missing in Iran six years ago was not a U.S. government employee.

Reports by The Washington Post and the Associated Press on the fate of Robert Levinson said the U.S. spy agency had been paying the former FBI agent to gather intelligence.

Experts from Iran, world powers and the U.N. atomic watchdog discussing how to implement November's nuclear deal are set to fall short of agreeing a start date for Tehran's six-month nuclear freeze, diplomats said.

The meeting between Iran and the United States, China, Russia, Britain, France, Germany – the so-called P5+1 -- was making progress however, they said.