The chief of Russia's space agency said that the United States has rejected a proposal for the two countries to explore the moon together, a Russian news agency reported.
A NASA spokesman in Washington said the U.S. was unaware of any Russian proposal, and was perplexed by the space chief's claim.
NASA announced in December that it would establish an international base camp on one of the moon's poles, permanently staffing it by 2024.
Officials with Russia's federal space agency, Roscosmos, later said they had hoped to join NASA's program, contributing Russian technology and space experience.
But Roscosmos chief Anatoly Perminov was quoted by the Interfax news agency Sunday as saying that the United States had rebuffed the offer.
"We are ready to cooperate but for some reasons the United States has announced that it will carry out the program itself," he was quoted as saying on Russian television.
"Strange as it is, the United States is short of experts to implement the program," he said.
Michael Braukus, a NASA spokesman in Washington, said the agency had received no lunar exploration proposal from Roscosmos.
"We haven't rejected anyone's proposal," he said. "We are really into involving the international community in this ... We're very interested in getting more involved and cooperating in our exploration efforts."
Perminov also said Russia had signed a $1 billion contract with NASA for Russian cargo ships to deliver goods to the international space station over the next three years — an indication, he said, of the competitiveness of Russia's space services.
"If we had been uncompetitive, such contracts would not be signed," Perminov was quoted as saying.
Russian space craft have been the workhorses of the international space station program, regularly shuttling cargo and people to the orbiting station — in particular after the U.S. space shuttle fleet was grounded following the Columbia disaster in 2003.
NASA will end the shuttle program in 2010 with plans to return to the moon in a new vehicle.