The ISS is a joint project of space agencies from the United States, Russia, Europe, Canada and Japan. The orbital station is likely to remain operational until 2020.
"So far, we have allocated about $4.2 billion for the ISS project, but we will need an additional $5 billion to finish the construction [of the Russian segment] by 2015," Energia President Vitaly Lopota said at a conference dedicated to Cosmonautics Day, celebrated in Russia on April 12.
The first ISS element, the Russian Zarya module, was put into orbit in November 1998. Several modules have been attached to the structure since then, but ongoing construction has missed the initial deadline, largely due to delays in U.S. shuttle launches and a lack of financing.
The Russian segment of the orbital station should have 10 modules by 2015, the Energia official said.
Russia is planning to launch two small research modules to the ISS in 2009-2010, a multi-purpose laboratory module - in 2011, and two energy modules - in 2014-2015.
The ISS crew will comprise six members, including three Russian cosmonauts, starting from 2009, Lopota said.