MOSCOW (AP) - U.S. Secretary of State Hillary Rodham Clinton and her Russian counterpart clashed openly Thursday over the planned launch this summer of Iran's first, Russian-built nuclear power plant, highlighting a split in views over how to steer Iran away from nuclear weapons.

Clinton did not criticize the long-delayed project directly but said the Obama administration is opposed to the timing of the nuclear plant's startup. Russian Prime Minister Vladimir Putin announced the summer startup plans on Thursday, shortly after Clinton arrived for a two-day visit.

The nuclear plant is an example of Russian-Iranian economic ties and technical cooperation, on terms that have long made the United States uncomfortable. It was a background issue during a difficult period in U.S.-Russian relations last year and in the ongoing U.S.-led effort to bring new United Nations economic penalties against Iran over suspicions that part of its nuclear program is aimed at building a bomb.

Putin's announcement adds another complication to the already long list of issues on which Clinton and her Russian hosts don't agree. Clinton is seeing Putin on Friday.

At a news conference with Foreign Minister Sergey Lavrov after talks on a wide range of issues, Clinton told reporters that Iran, while entitled to the peaceful use of nuclear energy, must reassure the world that it is not trying to build a nuclear weapon.

"In the absence of those reassurances, we think it would be premature to go forward with any project at this time, because we want to send an unequivocal message to the Iranians," she said.

Lavrov forcefully asserted that, whatever the U.S. concerns, his country will finish its work on the Bushehr nuclear power plant shortly.

"The project will be completed," Lavrov said. "We are now in the final stage, and this nuclear power plant will be launched. It will be put into operation, it will be functioning, producing power." He added that the plant will operate under strict compliance with requirements of the U.N.'s nuclear watchdog agency.

Lavrov and Clinton also asserted to reporters that U.S.-Russian negotiations on a new treaty to reduce long-range nuclear weapons are close to completion. The accord would replace the 1991 Strategic Arms Reduction Treaty, or START, which expired in December.

Lavrov said the two sides now are discussing the time and place for President Barack Obama and President Dmitry Medvedev to sign the new deal, which also must be ratified by each country's legislature. A Clinton spokesman, P.J. Crowley, said later that the negotiators were "down to one or two issues" before completing the deal.