The Arlington, Virgina-based firm, which brokered the first two tourist space flights in 2001 and 2002, has signed a contract with the Russian space agency Rosaviakosmos to fly two more tourists to the ISS in 2004-2005.

Eric Anderson, chief executive officer, told the news agency in Los Angeles that Space Adventures was keen to send the two space tourists on board one spaceship accompanied by a professional astronaut.

They even considered the possibility of buying a Soyuz spaceship. Negotiations on the general issue are underway, but it was not clear what the outcome of the talks would be, Andersen said.

A Russian space agency official said earlier this week that Russia could build and sell a Soyuz craft to a private company offering tourists flights but only for a very high price.

"We can sell a spaceship. They have to understand, however, that it will cost not 20 million or 40 million dollars, but much more," said Rosaviakosmos spokesman Sergei Gorbunov.

"Russia may build a Soyuz outside the ISS programme. This technical possibility exists, but we have not discussed anything yet or received any offers," he added.

The Russian space official also said that it was possible that the two future tourists could fly up to the orbiting space station on a single Soyuz -- leaving just one seat for a professional astronaut on the three-person craft.

The asking price for a 10-day visit to the ISS is around 20 million dollars (17.6 million euros), a sum only two space tourists -- Californian businessman Dennis Tito and South Africa Internet millionaire Mark Shuttleworth -- have so far been willing to pay.

Both tourists jaunts were organised by Space Adventures.

Russia and the United States, the major partners in the 16-nation ISS project, have clashed in the past over Moscow's keenness to raise money for its cash-strapped space program by selling tourist tickets to the ISS.

Russia sends Soyuz rockets to the ISS every six months on so-called "taxi missions."

SPACE.WIRE