The UK and US governments have warned Russia not to use humanitarian assistance as a pretext for sending troops into eastern Ukraine.
Any such intervention would be "completely unacceptable and "viewed as an invasion of Ukraine", said the US Ambassador to the UN, Samantha Power.
The UK Foreign Secretary, Philip Hammond, said: "I strongly urge Russia to avoid any provocative actions".
His office cited reports that Russian military vehicles had entered Ukraine.
Heavy fighting continued on Friday in eastern Ukraine, near the Russian border, between Ukrainian government forces and pro-Russia militants.
Mr Hammond said: "I am deeply concerned by reports of an increased flow of heavy weapons crossing into Ukrainian sovereign territory from Russia and by reports of Russian armed forces exercising for a 'humanitarian intervention' in a third country.
"The conditions for such an intervention in eastern Ukraine manifestly do not exist. In these circumstances, such an intervention would be unjustified and illegal."