Russia was modernizing a booster which US space agency NASA would use for manned flights, a Russian rocket engine manufacturer said Wednesday.

"We are adjusting the RD-180 engine for manned missions," Executive Director of NPO Energomash company Vladimir Solntsev told reporters, adding the adaptation work started in early 2012.

The RD-180 engine, which was supplied to the United States in 1999, was an upgraded version of the RD-170 booster used in Ukrainian Zenit rockets and Russian "Energy" carrier rockets, he said.

The upgraded RD-180 could put into orbit a workload of 400 tons, the company said.

Solntsev also revealed his company had been designing even more powerful space boosters with a 500-ton capacity.

"We are going to market it for our western partners," he said.

NPO Energomash is the leading Russian developer of liquid propellant rocket engines.