Foreign Secretary Philip Hammond has said he is "very alarmed" at reports that Russian military vehicles may have crossed into Ukraine and warned that there could be serious consequences.
A convoy including armoured personnel carriers were reported to have crossed the border near the Russian town Donetsk and Mr Hammond urged Moscow to withdraw its forces immediately.
The reports came as Russia let Ukrainian officials inspect a controversial aid convoy which it had despatched for distribution around the rebel-held city of Luhansk.
As European Union foreign ministers met to discuss the situation, Mr Hammond said: " I'm very alarmed by reports that Russian military vehicles may have crossed the border this morning.
"Of course the humanitarian convoy itself is a separate issue. But if there are any Russian military personnel or vehicles in eastern Ukraine they need to be withdrawn immediately or the consequences could be very serious."
As he arrived at the meeting with EU counterparts in Brussels Mr Hammond said the situation was "potentially very dangerous".
Tensions between the West and Russian president Vladimir Putin have escalated following the apparent shooting down of Malaysia Airlines flight MH17 by Moscow-backed separatists.
Mr Hammond said: "We would urge the Russians to withdraw any military personnel that they have inside Ukraine and stop supporting the separatists as they have been doing for the last few months.
"The situation is potentially very dangerous and that's why we are here today discussing it."