The Kremlin said on Friday that Moscow's move to help Beijing build an early warning system to detect missile attacks showed the two countries had a special relationship.

President Vladimir Putin said on Thursday that Russia was helping China build an early warning system to spot intercontinental ballistic missile launches, something only Russia and the United States possess at the moment.

"This is a very serious thing that will radically increase China's defense capability," Putin said.

Kremlin spokesman Dmitry Peskov declined to say when the system would be operational, but told reporters on a conference call that the move highlighted Russia's close ties with China.