Western leaders have stepped up pressure on Russia to abandon efforts to annex Ukraine's Crimea region or face sanctions.
The G7 leaders say they will not recognise the results of a referendum in Crimea this weekend on whether to split from Ukraine and join Russia. They say the referendum can have no legal effect becuse it is in direct violation of Ukraine's constitution.
The G7 nations (Britain, Canada, France, Germany, Italy, Japan and the United States) and the European Union have warned that any attempt to change Crimea's status through the referendum will have grave implications - and if Russia takes such a step they will "take further action, individually and collectively".
Separately, American president Barack Obama - speaking after holding talks with Ukraine's interim prime minister Arseniy Yatsenyuk in Washington - said the Western powers will be "forced to apply costs" if Russia does not change course.
Mr Yatsenyuk told reporters Ukraine "is and will be part of the Western world".
Earlier, Ukrainian security chief Andriy Parubiy warned of a major Russian military build-up on Ukraine's borders. He said Russian troops had not withdrawn since carrying out military exercises near Ukraine's eastern and southern frontiers last month, and were now "only two to three hours" from Kiev.