By JAY SOLOMON And RICHARD BOUDREAUX

MOSCOW—Prime Minister Vladimir Putin said Thursday that Russia will help Iran launch its first nuclear power plant this summer, delivering a diplomatic slap to visiting U.S. Secretary of State Hillary Clinton and a blow to U.S.-led efforts to increase financial pressure on Tehran.

Mr. Putin’s announcement, made at a conference on Russia’s nuclear-power industry in the southern city of Volgodonsk, took Mrs. Clinton’s entourage by surprise and drew a swift rebuke. Mrs. Clinton is on a two-day visit to Moscow to work, in part, on forging a united front with Russia on addressing Iran’s continuing push to develop nuclear technologies.

“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,” Mrs. Clinton said at a news conference with Russian Foreign Minister Sergei Lavrov following Mr. Putin’s comments.

Russia has been developing light-power reactors at the Bushehr complex for more than a decade, and the U.S. has supported the project in principle. But Russian analysts said Mr. Putin’s announcement threatened a tacit understanding with the West that Russia wouldn’t deliver the fuel to launch the Bushehr plant until Iran agreed to cooperate with international nuclear inspections.

Mr. Putin’s comments come as the Obama administration has endured other slights on the global stage in recent weeks. Israel’s government announced new construction in disputed East Jerusalem during a visit by Vice President Joe Biden last week. Chinese officials have rebuffed U.S. calls for a revaluing of the yuan and greater Internet freedoms.

Mr. Putin, in putting his stamp on a key foreign-policy issue, also appeared to undermine his handpicked successor as president, Dmitry Medvedev, and revive questions of who is calling the shots in Russia. Mr. Medvedev, whose office includes leading the country’s foreign policy, had indicated Russia would cooperate on putting pressure on Iran.

Russian officials attending the conference with Mr. Putin said Bushehr could be launched by July. Mr. Putin described the Iranian project as central to Moscow’s push to become a dominant international player in developing civilian nuclear power.

“We continue to work on developing atomic energy capacity both at home and abroad,” Russia’s Interfax news agency quoted Mr. Putin as saying.

President Barack Obama’s administration is pushing to ratchet up international pressure on Iran’s economy, particularly its energy sector, in a bid to contain Tehran’s nuclear program. The U.S. and others believe Iran is pursuing nuclear weapons. Iran says its nuclear ambitions are peaceful.

“We have consistently said that Iran is entitled to civil nuclear power,” Mrs. Clinton said at the news conference. “It is a nuclear-weapons program that it is not entitled to.”

Mrs. Clinton’s visit was part of a U.S. effort to improve relations with Russia. President Obama last year promised a “reset” of Washington’s ties with Moscow, which were often strained in the George W. Bush administration.

U.S. officials have voiced optimism in recent weeks that Moscow would support a tougher sanctions resolution on Tehran at the United Nations Security Council.

The U.S. also has believed Russia could emerge as an ally in gaining support for the resolution from Beijing. China, which has shown no sign of supporting the sanctions, has veto power on the Security Council.

Russia’s moves to push ahead with the Bushehr plans appear to undermine that.

Ivan Safranchuk, a consultant on Central Asia at the Moscow State Institute of International Relations, said Mr. Putin appeared to be pushing back against Western pressure—for Russia to either endorse tougher sanctions, or acquiesce to the implicit threat by the U.S. and Israel to take military action against Iran if sanctions fail.

“What Putin is signaling is that Russia does not accept this choice of harder sanctions or military means,” Mr. Safranchuk said.

Other U.S. officials sought to play down any split between Washington and Moscow. The U.S. has supported Moscow’s position that it has established the necessary safeguards to prevent Tehran from using Bushehr’s nuclear fuel for military purposes. Russia has committed to supplying low-enriched uranium for the plants and taking away its spent fuel rods. Inspectors from the United Nations will monitor the plant.

“We don’t have an issue with the Bushehr project overall,” State Department spokesman P.J. Crowley said. Mrs. Clinton’s “concern was simply: What message does this send more broadly as we’re trying to apply pressure on Iran?”

The State Department said Thursday that Mrs. Clinton would meet Mr. Putin late Friday in Moscow following her meeting with Mr. Medvedev.

The Putin meeting wasn’t initially on Mrs. Clinton’s schedule, and U.S. officials said the Russian prime minister made his availability known only Thursday morning.

Mrs. Clinton and Mr. Lavrov said Thursday that they expected their negotiators to complete a new strategic arms reduction treaty, which regulates the two nations’ nuclear-weapons stockpiles, in the coming weeks. And they touted cooperation between the U.S. and Russia in the war in Afghanistan.

“We believe that this reset of the relationship has led to much greater cooperation, coordination, and a constructive ongoing consultation on numerous issues that are important to our bilateral relationship and to the global issues that we both are facing,” Mrs. Clinton said.