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"If there are any Russian military personnel or vehicles in eastern Ukraine, they need to be withdrawn immediately or the consequences could be very serious."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
After the thundery activity overnight, clears skies follow leading to a sunny, humid day. Then more thundery outbreaks to come.
A British Army nurse is to be awarded the OBE for her role in the "vanguard" of efforts to control the deadly Ebola disease in Sierra Leone.
Leaders in France and Britain today pledged to work together and increase funds to help stop migrants smuggling their way across the Channel