U.N. nuclear inspectors have begun a mission to Iran to investigate allegations of secret military dimensions to the Iranian nuclear program.

The International Atomic Energy Agency team led by Herman Nackaerts arrived in Tehran on Sunday for a three-day visit. Before Nackaerts left the agency's headquarters in Vienna, he said he wants Iran to “engage” his team on the accusations that Iranian authorities are trying to develop nuclear weapons under cover of a peaceful energy program.

Iran has repeatedly denied the allegations and refused to discuss them with the IAEA or world powers. Nackaerts, a Belgian who serves as IAEA deputy director general, said his team's planned dialogue with Iran on issues of concern to the agency is “long overdue.”

It is not clear which sites the U.N. nuclear inspectors will visit in Iran or which Iranian officials they will meet.

The IAEA issued a report last November saying it has evidence that Iran engaged in nuclear activities related to developing nuclear weapons. Iran dismissed that report as based on fabricated intelligence from Western powers.

The United States and European Union have tightened economic sanctions on the Iranian central bank and oil industry in recent weeks to pressure Iran into stopping uranium enrichment – a process that has military and civilian applications. Earlier this month, Iran said it started enriching uranium at its underground Fordo complex, which has the capacity to speed up the production of weapons-grade material.

Iran has threatened to respond to Western sanctions by closing the Strait of Hormuz, a vital pathway for global oil supplies. Iranian officials also have warned that an embargo on Iranian crude exports will raise oil prices and hurt the West more than Iran.

On Sunday, Iranian state media quote the head of Iran's state oil company Ahmad Qalehbani as saying he expects an increase in oil prices to a range of $120 to $150 a barrel. He did not give a timeframe for the prediction. U.S. oil futures closed at $99.56 a barrel on Friday.