The United States is again expressing concern about Russian nuclear cooperation with Iran - this as a Russian company shipped to Iran Friday a key component for a nuclear power plant under construction on the Persian Gulf.

The U.S. comments followed a ceremony Friday in St. Petersburg in which a Russian company officially sent off to Iran a 300-ton steel reactor body for the Bushehr power plant being built, largely by Russian technicians, along the Persian Gulf coast.

Though both Russian and Iranian officials insist the plant will be used only for civilian purposes under international inspections, State Department spokesman Philip Reeker reiterated U.S. concerns that the plant will give Iran both technical know-how and fissionable material to advance a secret nuclear weapons effort.

"We believe Iran uses Bushehr as a cover for obtaining sensitive technologies to advance its nuclear weapons program," he said.

Mr. Reeker said he did not know if the issue was raised during President Bush's summit talks this week with Russian President Vladimir Putin, but said the United States has many channels through which to make its objections to the nuclear project known to Moscow.