"If no-one feels like extending the agreement -- New START -- well, we won't do it then," Putin said at an economic forum in Saint Petersburg, referring to the Strategic Arms Reduction Treaty.

"We said a hundred times that we are ready (to extend it)," Putin said, lamenting that Washington was "not conducting any talks" on the issue with Moscow.

"There is no formal negotiating process. And in 2021 everything will end."

The treaty was signed by US President Barack Obama and Russian counterpart Dmitry Medvedev in Prague in 2010.

The agreement, which caps the number of nuclear warheads well below Cold War limits, is set to expire in two years' time.

Together with another agreement known as the Intermediate-Range Nuclear Forces (INF) treaty, New START is considered a centrepiece of superpower arms control.

Washington and Moscow dropped the INF treaty earlier this year amid mutual recriminations.

Putin said the potential implications of letting the other treaty expire would be huge, suggesting its demise would fuel a nuclear arms race.

"There won't be any instruments limiting an arms race, for example, deploying space-based weapons."

"This means that nuclear weapons will be hanging over every one of us all the time."

At the same time, the president said Russia would not be afraid of shelving the treaty because it was developing a new generation of weapons that will "ensure Russia's security" in the long term.

"When it comes to creating hyper-weapons, we have overtaken our competitors," the Russian president said.

In February, Washington pulled out of the INF treaty, which was signed in 1987, over what it said were Russian violations of the terms.

Russia then followed suit.