US President Barack Obama warned after meeting Monday with German Chancellor Angela Merkel on the Ukraine crisis that the West cannot allow Russia to redraw Europe's borders "at the barrel of a gun."

"We continue to encourage a diplomatic resolution to this issue," Obama told a joint press conference.

"And as diplomatic efforts continue this week, we're in absolute agreement that the 21st century cannot stand idle, have us stand idle and simply allow the borders of Europe to be redrawn at the barrel of a gun."