In the speech given to Russia's diplomatic corps in Moscow, Medvedev said that US plans to site a missile defense system in Eastern Europe threatens the demise of the decades-old balance of power in Europe and added that Russia was prepared to execute "an adequate response" should the plans advance.
"This common (security) heritage cannot survive if one of the sides destroys isolated elements of the strategic construction," Medvedev said in the sharply-worded speech which had echoes of Vladimir Putin at his most beligerent. "We are unsatisfied…National security cannot be based on spoken promises.
"These installations ... only worsen the situation," he added. "We will be forced to respond to this adequately. The EU and US have been warned."
Russia pulled out of a key Soviet-era treaty limiting troop movements and arms buildups across the continent in the fall after the souring of negotiations on US missile plans and over US backing for the expansion of NATO forces along its border.
Russia's top military brass resent these moves as a continuation of the United States' Cold War policy of containment.
The defeat of the Conventional Armed Forces Treaty in Europe (CFE) which provided for the monitoring of Russian troop movements has set European defense experts on edge.
"We would not like to think that only (the treaty's) final collapse would prove to everyone that the existing security regime is ineffective and unfair," Medvedev said.
Missile shield worsening European defense
Moscow says US missile defense plans for Poland and the Czech Republic undermine its nuclear deterrent and that Washington has failed to convince it of the system's necessity to protect against threats from "rogue states" such as Iran.
"We firmly declare that the deployment of elements of a US global anti-missile system in Eastern Europe only worsens the situation. We will be forced to react in kind to this threat," Medvedev said.
Last week the Czech Republic agreed to host radar installations for the planned US missile defense system, sparking an angry reaction from Moscow.
Russia attacks West's control of smaller states
Medvedev also attacked Western "paternalism", accusing Russia's former Cold War foes of making decisions for less powerful nations.
"With the end of the Cold War, there is no reason to have a bloc mentality. There is also no reason for paternalism, where some countries decide everything for others," Medvedev said.
Medvedev also criticized Kosovo's Western-backed independence declaration and likened the situation there to Iraq.
"For the EU, Kosovo is almost what Iraq is to the United States.... This is the latest example of the undermining of international law," he added.
Moscow says Kosovo's self-rule is illegal since the region's independence was not condoned by Serbia, a traditional Russian ally.
The United States and a number of European nations have recognized Kosovo's independence despite protests from Russia and Serbia.
On a more conciliatory note, Medvedev told the group of diplomats that they should refrain from Cold War-style confrontation in international affairs. "We have to defend our national interests correctly and without confrontation," he said.
Medvedev following Putin's path of assertiveness
Medvedev's comments suggested he would maintain the assertive stance of his predecessor Vladimir Putin, now prime minister, who as president often lashed out at Western policies in Moscow's former Soviet-era sphere of influence.
His speech at the foreign ministry conference, an event held every two years, was also billed by officials as bolstering his credentials on foreign policy amid assertions by analysts that Putin still wields a major role.
Prior to Medvedev's inauguration in May, some Western observers had expressed hope that the new Russian leader might adopt a more conciliatory posture in international affairs than Putin.
But since becoming president, Medvedev has set about asserting Russia's position as the powerful neighbor on Europe's doorstep and protecting its interests on the international stage by flexing its considerable political and military muscle.