They said if Russia took such a step they would "take further action, individually and collectively".
The G7 leaders also said they would not recognise the results of a referendum in Crimea this weekend on whether to split from Ukraine and join Russia.
Meanwhile, Ukraine's national security chief has warned of a major Russian military build-up on Ukraine's borders.
Andriy Parubiy said Moscow had not withdrawn its troops after carrying out military exercises near Ukraine's eastern and southern frontiers.
"The Russian army is only two to three hours' march from Kiev," he said, adding that Ukraine's "units are positioned to repel attack from any direction." (+ BBC video) » | Wednesday, March 12, 2014
