(CNN) -- Top International Atomic Energy Agency officials arrived in Iran Sunday, state media reported, after the head of the United Nations nuclear watchdog expressed fresh concerns that the Islamic republic was trying to develop nuclear weapons.

The six-member delegation, including chief inspector Herman Nackaerts, arrived at Tehran's Imam Khomeini International Airport, Press TV reported.

"We are trying... to resolve all the outstanding issues with Iran," Nackaerts told reporters earlier, as he was about to leave Vienna, Austria, according to Press TV. "We are looking forward to the start of a dialogue, a dialogue that is overdue."

A mission to Iran by such a senior team -- which also includes the agency's second-in-command, Rafael Grossi -- is unusual, the agency said when it announced the visit on Monday. The team is due to be in Iran through Tuesday, the IAEA says.

New sanctions to push Iran to negotiate

The announcement of the mission came shortly after the European Union imposed a tough round of new sanctions on Iran, aimed at cutting off funding to the country's nuclear program. The United States and Australia have also ramped up sanctions on Iran in the past week.

The United States and its allies think Tehran is trying to develop nuclear weapons. Iran denies it.

Speaking Friday at the Davos Forum in Switzerland, energy agency Director General Yukiya Amano told reporters the visit is intended "to clarify the issues with possible military dimensions.

"We are not very sure whether Iran has declared everything and, therefore, we are not very sure that everything stays in peaceful purpose," he said. "In addition, we have information that Iran has engaged in activities related to the development of nuclear weapons. Therefore, we need to clarify."

"The preparations have gone well, but we need to see what actually happens when the mission arrives," he said.

Inspectors are in and out of the country regularly, an agency spokeswoman said Monday, but a high-level visit of the kind taking place at the end of the month is more unusual.

Iran's envoy to the energy agency said Saturday he was hopeful the trip will "resolve any ambiguity and show (our) transparency and cooperation with the agency."

"This trip is aimed at neutralizing enemy plots ... and baseless allegations, and proving the peaceful nature of our nuclear activities," Ali Asghar Soltanieh told state-run Islamic Republican News Agency.

Amano said that the energy agency proposed the mission, and Iranian authorities "agreed to accept" it. But the Islamic news agency reported Nackaerts is traveling at Tehran's invitation.

Iran's President Mahmoud Ahmadinejad has said he was ready to discuss the program with a group of world powers that have been having on-and-off negotiations with the country over its nuclear ambitions -- including Britain, China, France, Germany, Russia and the United States.

The energy agency reported in November that it can no longer verify that the Iranian nuclear program remains peaceful, and Iran is under increasing international pressure to halt its nuclear fuel work.

Western sanctions have targeted its currency, the rial, driving up prices for goods within Iran, and the European Union announced Monday that it would stop importing Iranian oil as of July 1 in an effort to starve the country's nuclear program of funding.

In response, an Iranian official said Sunday that Tehran would stop oil exports to "certain countries," soon, according to the state-run Islamic Republic News Agency. Oil Minister Rostam Qassemi made the statement after a cabinet meeting Sunday, as Iranian lawmakers are debating whether to halt oil exports to European countries, IRNA reported.