Russia says it is prepared to use "destructive force pre-emptively" if the US goes ahead with controversial plans for a missile defence system based in Central Europe.
The warning came after the Russian defence minister said talks on missile defence were nearing a dead end.
Moscow fears that missile interceptors would be a threat to Russia's security.
But the US and Nato say they are intended to protect against attacks from Iran or North Korea.
"A decision to use destructive force pre-emptively will be taken if the situation worsens," chief of the Russian defence staff Gen Nikolai Makarov said.
'Flawed assumptions'
Two days of talks opened on Thursday in Moscow between Russia, the US and Nato.
Russian Defence Minister Anatoly Serdyukov said the talks were "close to a dead end", but Nato said it remained hopeful of reaching a deal.
Nato Deputy Secretary General Alexander Vershbow told the BBC that Russia's fears of a European missile defence shield were "based on some flawed assumptions" and did not weaken Russia's nuclear deterrent.
Gen Makarov also said that if the European shield was built, Russia would respond by putting more powerful warheads on its own ballistic missiles.
Russia and the US have been at odds over the issue of missile defence since 2000, ever since the idea was first put forward by then-President George W Bush.
President Barack Obama, who succeeded Mr Bush in the White House in 2008, scrapped plans for a network of bases spread across Poland and the Czech Republic with the capacity to intercept long-range missiles.
But in 2010, it signed an agreement with Poland to use an old airstrip at Redzikowo, near the Baltic coast, as a missile defence base.
For its part, Russia has put into commission a radar system in its Baltic enclave of Kaliningrad which is capable of monitoring missile launches from Europe and the North Atlantic.
