Foreign Secretary Philip Hammond said he is "alarmed" by reports that Russian military vehicles crossed into Ukraine overnight, suggesting it could have "very serious consequences".
Reports suggest more than 70 pieces of military hardware have crossed the border, as Ukrainian officials began to inspect trucks containing humanitarian aid heading to eastern Ukraine.
"I'm very alarmed by reports that Russian military vehicles may have crossed the border this morning," he told reporters at a European Union foreign ministers' meeting.
"If there are any Russian military personnel or vehicles in eastern Ukraine, they need to be withdrawn immediately or the consequences could be very serious."
More top news
Ridge of high pressure will settle conditions for many tomorrow, however from the west more rain will arrive and the wind will pick back up
It has caused chaos on the roads and rail network, brought down trees, grounded planes, toppled lorries and also a bus in Cambridgeshire.
The nature of Ian Stewart's killing of his fiancee Helen Bailey and plot to evade detection raised doubts about the death of his wife Diane.