Leaders of the G7 group of nations have called on Russia to stop all efforts to “annex” Ukraine’s Crimea region.
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.
Separately, the US president said they “will be forced to apply costs” if Russia does not change course.
Barack Obama was speaking after holding talks with Ukraine’s interim Prime Minister Arseniy Yatsenyuk in Washington.