Ukraine crisis: 'International monitors seized' in Sloviansk
25 April 2014 Last updated at 22:30 BST
Pro-Russian separatists in eastern Ukraine have seized a bus carrying international military observers, Ukraine's interior ministry says.
Negotiations were under way to secure the release of the observers, seized near the town of Sloviansk, it added.
Ukraine warned it would consider any crossing into its territory by Russian troops as a "military invasion".
Daniel Sandford reports.