The United States has imposed sanctions on five Iranian nuclear experts and seven companies for helping Iran move closer towards enriching uranium — essential for building a nuclear bomb.

The new sanctions freeze the U.S. assets of those named and forbids any U.S. citizen or body from doing business with the 12.

Iran has refused U.N. and Western demands that it give up its nuclear program, which Washington says is aimed at building a bomb. Iran insists its nuclear activities are strictly for peaceful purposes

U.S. State Department spokeswoman Victoria Nuland says because of Iran's continued intransigence on its nuclear program, it is essential to restrain Iranian violations.

Meanwhile, a team of United Nations nuclear inspectors is in Tehran seeking to convince Iran to resume nuclear talks.

Chief inspector Herman Nackaerts said the focus of the visit is to resolve outstanding issues related to Iran's possible development of nuclear weapons.

The U.N. nuclear agency wants Iran to grant inspectors access to the Parchin military installation southwest of Tehran, which the West suspects is related to possible nuclear weapons development. Iran says Parchin is a conventional military site.