The Collective Security Treaty Organization (CSTO) agreed on Wednesday at a summit in Moscow to set up the new force, to be based in Russia.
Medvedev said the force, to be comprised of a "sufficient" number of units, would be "well trained and well equipped."
"Russia is ready to contribute a division and a brigade," he said. "This gives you an idea of the scale."
The Russian president also said the CSTO was open for cooperation with the United States in the fight against terrorism in Central Asia.
The Collective Security Treaty Organization (CSTO) is a security grouping comprising the former Soviet republics of Armenia, Belarus, Kazakhstan, Kyrgyzstan, Russia, Uzbekistan and Tajikistan.
The rapid-reaction force agreement was signed by the leaders of all CSTO member states in the Kremlin, although Uzbekistan recorded "a special opinion."
"Uzbekistan cannot accept the provision whereby all special services, including emergency services, are to be part of the collective force," CSTO press secretary Vitaly Strugovets said.
The force will be used to repulse military aggression, conduct anti-terrorist operations, fight transnational crime and drug trafficking, and neutralize the effects of natural disasters.
The force will be permanently based in Russia and placed under a single command, with CSTO member countries contributing special military units.
A source in the Russian delegation said Uzbekistan would not participate in the collective force on a permanent basis but would "delegate" its detachments to take part in operations on an ad hoc basis.