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"If there are any Russian military personnel or vehicles in eastern Ukraine, they need to be withdrawn immediately or the consequences could be very serious."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
This evening it'll become largely dry across the UK as the showers in the west gradually die down.
Multiple fatalities have been reported after an attack on a centre for disabled people near Tokyo, Japanese media reports suggest.
In a blog, basketball great Michael Jordan spoke out about the recent killings of both unarmed black men and police in the US.