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.”
Earlier, the Russian defence ministry said military exercises in southern Russia had ended and that forces were returning to their permanent bases.
Nato says Russia has massed about 20,000 troops near the border with Ukraine.
Western sanctions against Russia over its role in events in Ukraine brought retaliation from the Kremlin this week, with a ban on most food imports from the US and EU.
Ukraine officers arrested
Russian investigators have arrested five Ukrainian army officers, accusing them of using heavy weapons and killing civilians in their operations in Ukraine’s Luhansk region.
They were among 400 Ukrainian soldiers who crossed the border to escape rebel fire on Sunday.
It is unclear on what grounds Russia can prosecute the officers, who reportedly deny the charges.
The Ukrainian government said 15 of its soldiers were killed and 79 injured on Thursday.
Government forces have been accused of bombarding residential areas from the ground and the air, killing civilians, as they try to recapture rebel strongholds. International monitors have been surveying damage to homes.
In a statement (in Russian) on its website, Russia’s powerful Investigative Committee (SK) announced that the five Ukrainian officers had been detained following questioning of 400 soldiers from the 72nd Brigade and Ukrainian border guards.
The Ukrainian service personnel were housed by Russian border guards in a tent camp near Gukovo, in Russia’s Rostov region, after surrendering their weapons. Many have since returned to Ukraine under Russian escort.
The five were named as battalion commander Ivan Voitenko and four of his subordinates – Vitaliy Dubyniak, Olexander Poliakov, Olexander Ohrimenko and Dmytro Ustylko.
Around 1,500 people, both civilians and combatants, have been killed since Ukraine’s new government sent forces into the east in April to put down an armed uprising by the separatists.