The White House joined Kiev tonight in accusing Russia of a "flagrant violation of international law" after a convoy of lorries carrying humanitarian aid crossed the border with Ukraine without permission.
It said Russia should remove them immediately, or face further economic sanctions.
ITV News' Europe Editor James Mates reports from Washington:
More top news
Other entries in the UKTV channel Gold competition included jokes about Donald Trump, Brexit, Southern Rail and Ryanair UKTV channel Gold.
Temperatures will remain close to freezing.
The prime minister's announcement came as international leaders gather in Paris on the second anniversary of the climate change plan.