Russian space officials plan to ignore a request to delay the launch of a three-man crew including the world's first space tourist to the International Space Station.
The US space agency had asked for the flight to be postponed from tomorrow.
Computer problems forced astronauts on the ISS to put off operations of a critical robot arm and Nasa would like to extend the space shuttle Endeavour's stay at the station for a second day.
But that would complicate the docking of Russia's Soyuz spaceship carrying US businessman Mr Dennis Tito and two cosmonauts.
The Soyuz is to blast off tomorrow and the docking is scheduled for Monday.
Mr Yuri Semyonov, the head of the state-controlled RKK Energia space corporation, said space experts concluded the Soyuz will be able to dock with the space station even while the shuttle is there.
But US shuttle flight director Phil Engelauf said the Russian craft would come "uncomfortably close" to Endeavour's tail on approach.
The hull of the Endeavour could also deflect the signal from the ISS's automatic docking system, potentially forcing the Soyuz crew to shift to the more dangerous manual docking, Russian space officials said.
Russia is unwilling to postpone the Soyuz mission because the cosmonauts need to replace the ISS's escape craft, whose service lifetime expires at the end of the month. "The Americans should solve the problems they're having on the ISS themselves," Mr Semyonov said.
The Soyuz space ship that brings Tito and the cosmonauts to orbit will serve as the new lifeboat and the three men will return on the older ship.