Russia has shipped the shell of a nuclear reactor to Iran as part of a one-billion-dollar deal that has angered the United States. The reactor is part of a new nuclear power station being built by Russian engineers in Iran.

The reactor shell was taken by train from a factory in St. Petersburg and later loaded onto a ship bound for Iran. The shell is the latest piece of equipment sent to Iran where Russian engineers are constructing the power station at Bushehr in the south of the country.

Russian specialists will travel to Iran to help install the reactor shell when it arrives in about one month.

Russian officials have repeatedly said the controversial nuclear facility will only be for civilian use. But the United States has long opposed the deal, saying Russia is supplying Iran with technology that could be used to produce nuclear weapons.

On Friday, U.S. State Department spokesman Philip Reeker again criticized Russia, saying that Iran is using the deal as a cover to advance its nuclear weapons program. The Bushehr reactor is expected to begin functioning in about two years.

Iran is included on the U.S. list of nations that the United States believes pose a threat to the rest of the world. Other countries on the list include Iraq and North Korea. Russia says it understands U.S. concerns but insists that the nuclear deal complies with all international norms.

Moscow and Tehran signed an agreement on the Bushehr plant in 1995, but it has been delayed in part because of the dispute with the United States. In the past, the United States has even warned Russia that it could face sanctions over its nuclear cooperation with Iran.

Yet the commercial aspect of the agreement is important for Russia. Iran has even placed a tentative order for a second nuclear reactor, with formal contract discussions to start soon.