'Humiliating for Russia'
Referring to Georgia and Ukraine's ambitions to join Nato, the Kremlin leader said: "Just by getting closer to Russia's borders, Nato is not becoming stronger ... what if Georgia had a Nato membership action plan?
"I would not wait for a second in making the decision I made at that point."
Medvedev said Georgian membership of Nato would be a destabilising factor, both for Nato and for the Caucasus region.
"The situation is not fair to Russia, it is humiliating for Russia. We are not going to tolerate this any longer," he said.
The Russian president balanced his remarks by saying he did not believe the Caucasus crisis had caused problems in relations between Russia and the West, which would lead to another long period of confrontation.
Regarding Iran's nuclear programme, Medvedev said a military solution to the standoff with Tehran was unacceptable, and there was no need at the moment for new sanctions against Iran.
"We should not take any unilateral steps. It is not acceptable to opt for a military scenario. It would be dangerous," he said.
The president said Moscow supported negotiations being led by Javier Solana, the EU foreign policy chief, to persuade Iran to curb its nuclear programme in exchange for a package of incentives.
"The key is that negotiations be pursued ... They have been quite positive. We should not adopt any additional sanctions now," he said.
Medvedev's comments followed remarks on Thursday by Vladimir Putin, Russia's prime minister, that there was no basis for a new Cold War with the West and amid concerns over Moscow's unilateral recognition of South Ossetisa and Abkhazia, Georgia's two separatist regions.
Putin, who was replaced by Medvedev as president in May, said Russia did not have any "imperial ambitions" but warned the West against starting an arms race in Europe by stationing the planned US missile system near Russia's borders.