More than 100,000 Russian troops have massed on Ukraine's border and in annexed Crimea, the office of the EU's top diplomat Josep Borrell said after EU foreign ministers were briefed by Ukraine's foreign minister.

In a news conference on Monday, Borrell had originally spoken of more than 150,000 troops, and declined to give a source for the figure.

His office later corrected the number to more than 100,000 troops without giving a reason for the change.