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"If there are any Russian military personnel or vehicles in eastern Ukraine, they need to be withdrawn immediately or the consequences could be very serious."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
The government has announced plans to ensure staff receive tips left by customers in bars and restaurants.
Tens of thousands are having do not resuscitate orders imposed without families' consent, according to the Royal College of Physicians.
The prime minister has said the debate over Britain's EU membership has affected his long-term friendship with the mayor of London.