Russia's top military officer has threatened to launch a pre-emptive strike if Nato goes ahead with controversial plans for a missile defence system in Eastern Europe.
President Dmitry Medvedev said last year that Russia will retaliate militarily if it does not reach an agreement with the United States and Nato on the plans.
Chief of General Staff Nikolai Makarov went even further today.
"A decision to use destructive force pre-emptively will be taken if the situation worsens," he said.
Russian Defence Minister Anatoly Serdyukov also warned that talks between Moscow and Washington on the topic are "close to a dead end".
US missile defence plans in Europe have been one of the touchiest subjects in US-Russian relations for years.
Moscow rejects Washington's claim that the missile defence plan is solely to deal with any Iranian missile threat and has voiced fears it will eventually become powerful enough to undermine Russia's nuclear deterrent.
Moscow has proposed running the missile shield jointly with Nato, but the alliance has rejected that proposal.
Makarov's statement does not seem to imply an immediate threat, but aims to put extra pressure on Washington to agree to Russia's demands.
The Obama administration tried to ease tensions with Russia in 2009 by saying it would revamp an earlier Bush-era plan to emphasise shorter-range interceptors. Russia initially welcomed that move, but has more recently suggested the new interceptors could threaten its missiles as the US interceptors are upgraded.
The US-Nato missile defence plans use Aegis radars and interceptors on ships and a more powerful radar based in Turkey in the first phase, followed by radar and interceptor facilities in Romania and Poland.
Russia would not plan any retaliation unless the United States goes through with its plans and take the third and final step and deploy defence elements in Poland, Deputy Defence Minister Anatoly Antonov has said. That is estimated to happen no earlier than in 2018.
Russia has just commissioned a radar in Kaliningrad, its western outpost near the Polish border, capable of monitoring missile launches from Europe and the North Atlantic.
Today, at the start of a two-day conference with representatives from about 50 countries, a top Russian defence official reiterated Moscow's offer to run the missile shield together with Nato.
Russia's Security Council Secretary Nikolai Patrushev said such a jointly run European missile defence system "could strengthen security of every single country of the continent" and "would be adequate to possible threats and will not deter strategic security."
Nato's deputy secretary general, Alexander Vershbow, told the conference that the US-led missile shield is "not and will not be directed against Russia" and that Russia's intercontinental ballistic missiles are "too fast and too sophisticated" for the planned system to intercept.
The conference in Moscow is the last major Russia-US meeting about military issues before a Nato summit in Chicago later this month.