Russia and China have condemned a US plan for a global missile defence shield, saying it will set back international disarmament efforts.
Russian President Dmitry Medvedev signed a joint statement with Chinese President Hu Jintao in Beijing.
The US says the shield, parts of which it plans to deploy in Europe, is key to its own security and that of allies.
Both Russia and China have opposed the plans in the past but this is the first time they have come together to do so.
The US, which wants to locate a radar base in the Czech Republic and an interceptor missile site in Poland, says the system will offer protection against missiles fired from states like Iran and North Korea.
However, the plans have soured US-Russian ties, with Moscow fearing that the system could be used against Russia.
The joint statement from Presidents Medvedev and Hu came as the Russian leader began a two-day state visit to China, part of his first foreign trip since he assumed office on 7 May
"Both sides believe that creating a global missile defence system, including deploying such systems in certain regions of the world, or plans for such co-operation, do not help support strategic balance and stability, and harm international efforts to control arms and the non-proliferation process," it said.
"It harms the strengthening of trust between states and regional stability. In this respect [the two sides] express their concern."
Russia last month demanded it be granted permanent access to the planned US sites in central Europe, saying it would need to monitor their activities.
This is the first time President Medvedev - who took office earlier in May - has openly criticised the US missile system in the style of his mentor and predecessor Vladimir Putin, correspondents say.
The tone of official Russian pronouncements on Friday, with the notion of "Western double standards", suggests no imminent thaw in Russia's relations with the West should be expected, the BBC's Russian affairs analyst Steven Eke says.
He says that Moscow and China again stressed their shared view of the world based on rejection of alleged American dominance.
The two countries have deepened their strategic co-operation in recent years, even holding joint military exercises, our analyst adds.
In Beijing, Mr Medvedev and Mr Hu also witnessed the signing of a $1bn (£500m) deal for Russia to build a nuclear fuel plant in China and supply semi-enriched uranium
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