Ukraine's Interior Ministry says armed pro-Russian separatists in the eastern city of Slovyansk have seized a group of international representatives of the Organization for Security and Cooperation in Europe (OSCE).
The ministry said negotiations were taking place for the release of the group, which includes seven OSCE representatives, five members of the Ukrainian armed forces, and a driver.
The April 25 statement said rebels had seized a bus carrying the observers from the OSCE. It said the group was being held in a State Security Service building held by the Slovyansk separatists.
The separatist mayor of Slovyansk, Vyacheslav Ponomaryov, told Reuters that the group had been detained because there were suspicions that a member of the Ukrainian government's military staff, whom he called a spy, was among the group.
OSCE observers have been in the region to oversee implementation of the Geneva agreement -- signed April 17 by Ukraine’s government, Russia, the United States, and the European Union -- aimed at de-escalating the situation in Ukraine.
Western states and Russia have accused each other of failing to take steps to implement the accord.
The White House, meanwhile, said President Barack Obama and the leaders of Britain, France, Germany, and Italy have agreed to "coordinate additional steps to impose costs" on Russia, such as sanctions, over its actions in the Ukraine crisis.
An April 25 White House statement said the leaders agreed during a conference call that Russia has "continued to escalate the situation through its increasingly concerning rhetoric and threatening military exercises on Ukraine’s border."
The statement accused Moscow of not publicly supporting the Geneva accord. It said Russia had failed to pressure pro-Russian armed militant groups to lay down their weapons and leave government buildings they occupy, in line with the Geneva agreement.
The statement added that "Russia could still choose a peaceful resolution to the crisis, including by implementing the Geneva accord."
Obama earlier warned that the United States was ready to impose sanctions on sectors of the Russian economy, such as banking or energy, if Russian forces crossed the border to invade eastern Ukraine.
Russia, however, on April 25 demanded that Ukraine stop "all military action" in eastern Ukraine.
The Foreign Ministry said Russia urges Ukraine to start implementing the Geneva accord.
Ukrainian forces this week have been in action in the east as part of what authorities have described as an “antiterrorist” operation against pro-Russian separatists.
Ukrainian Prime Minister Arseniy Yatsenyuk accused Russia of seeking to start World War III with its actions in Ukraine.
He said Russia aims to occupy Ukraine "militarily and politically" and spark a war in Ukraine that "will lead to a military conflict in Europe."
Russia has denied charges from Ukraine and Western states that it has sent operatives into Ukraine to stir up separatist unrest.
Based on reporting by Reuters, AP and AFP