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"If there are any Russian military personnel or vehicles in eastern Ukraine, they need to be withdrawn immediately or the consequences could be very serious."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
Armed police have taken part in a dramatic training exercise aimed at recreating the chaos of a Paris-style terror attack.
After a remarkably mild November - a notable mild start to the new month.
The Facebook founder revealed the plan as he and wife Priscilla Chan announced the birth of their baby daughter, Max.