US President Barack Obama. Star file photo
US President Barack Obama has urged Russia to stop "intimidating" Ukraine and reduce the number of troops it has on its border.
He also called on Russia to "de-escalate the situation" and begin negotiations with Kiev.
Russia has massed a force of some 30,000 troops close to the eastern frontier of Ukraine in recent days.
Obama told CBS News it may "be an effort to intimidate Ukraine, or it may be that [Russia has] additional plans."