The two days of talks come amid heightened tensions in the region, with Israel making clear it is pondering an attack on Tehran's nuclear infrastructure, while Iran warned it could cut off the narrow strait through which oil tankers sail in and out of the gulf.
The scheduled talks between the International Atomic Energy Agency and Iranian officials are billed as an opportunity for the watchdog agency to clarify the "possible military dimensions to Iran's nuclear program," the group said.
Iran says it is producing enriched uranium to fuel civilian power plants and has refused international demands to halt its production.
But the IAEA reported in November that it had information to suggest Iran had carried out some weapons-related research.
U.N. Secretary-General Ban Ki-moon said it is up to Iran to disprove the allegation.
"The Agency is committed to intensifying dialogue. It remains essential to make progress on substantive issues," IAEA Director General Yukiya Amano said in statement following the first round of talks in January.
The talks in Tehran follow an announcement Sunday by Iran's oil ministry that it was halting crude exports to French and British companies, an order that followed a threat that Iran would cut oil exports to some European Union countries in retaliation for sanctions put in place last month by the EU and the United States.
"Iran has no difficulty in selling and exporting its crude oil. ... We have our own customers and have designated alternatives for our oil sales. We shall sell to new customers, who will replace French and UK companies," ministry spokesman Ali Reza Nikzad-Rahbar said in a statement.
The sanctions put in place last month are meant to force Iran to provide more information on its nuclear program by shutting off its sale of crude oil, which generates half of Iran's revenue.
Iran exports 2.2 million barrels of oil a day, 18% of which is bound for European markets, according to the U.S. Energy Information Administration. The world consumes about 89 million barrels of oil per day.
U.S. and European sanctions are already squeezing Iran's economy, driving down its currency and driving up consumer prices.
Israeli Defense Minister Ehud Barak has called for still-tougher sanction, saying that accelerating the pace of sanctions would force Tehran to return to nuclear talks.
Iran proposed a resumption of those stalled talks last week. U.S. and European diplomats were still trying to gauge the sincerity of the Iranian offer, but U.S. Secretary of State Hillary Clinton called it "an important step."
Meanwhile, Israel has made that it considers a nuclear-armed Iran an existential threat, and has made clear it is considering an attack on Tehran's nuclear infrastructure.
The United States believes talk of military strikes against Iran's nuclear program is "premature" and has advised Israel that an attack would be counterproductive, Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, said Sunday on CNN's "Fareed Zakaria GPS."
Dempsey said U.S. officials aren't convinced Iran has decided to pursue nuclear weapons. Meanwhile, economic and diplomatic sanctions appear to be taking a toll on the Islamic republic, he said.
"On that basis, I think it would be premature to exclusively decide that the time for a military option was upon us," Dempsey said.
Dempsey said American officials believe an Israeli strike would delay Iran's nuclear development "probably for a couple of years, but some of the targets are probably beyond their reach." He said he and others have had "a very candid, collaborative conversation" with the Israelis about the issue.
"I'm confident that they understand our concerns, that a strike at this time would be destabilizing and wouldn't achieve their long-term objectives," he said. "But, I mean, I also understand that Israel has national interests that are unique to them. And, of course, they consider Iran to be an existential threat in a way that we have not concluded that Iran is an existential threat."
Tensions between Iran and Israel have been further heightened in recent days with two Iranian warships sailing through Egypt's Suez Canal into the Mediterranean Sea.
It's only the second time such ships have crossed through the Suez Canal since Iran's 1979 revolution.
Iran's Press TV reported on Monday that the two ships had docked Saturday in the Syrian port of Tartus where the crews were to provide maritime training to Syrian naval forces.
