Medvedev warns Ukraine over Nato
Mr Medvedev told Viktor Yushchenko at talks in St Petersburg that joining Nato would breach a friendship treaty, Russia's foreign minister said.
He also announced Russia would almost double the price of gas exports to Ukraine, blaming Central Asian costs.
The presidents are attending an informal summit of 12 ex-Soviet states.
Mr Medvedev also met Georgia's leader amid rising bilateral tensions.
The Western-leaning Mr Yushchenko has secured an assurance that Ukraine may be allowed to join Nato in the future.
The agreement has angered Russia, which is opposed to any further eastward expansion of the military alliance.