Ukraine crisis: Kerry urges Russia to help free observers
US Secretary of State John Kerry has urged Russia to do all it can to help release European military observers abducted in eastern Ukraine.
In a phone call with his Russian counterpart, Mr Kerry also expressed concern about "provocative Russian troop movements" near Ukraine's border.
Pro-Russia militia continue to occupy official buildings in a dozen eastern cities, defying the government in Kiev.
EU diplomats are to meet on Monday to discuss fresh sanctions against Russia.
The G7 group of economic powers has also agreed to intensify sanctions against Russia over Ukraine.
Speaking on a visit to Malaysia on Sunday, US President Barack Obama stressed it was important to take further steps to send "a message" to Moscow that its destabilising actions in eastern Ukraine must stop.
But Mr Obama stressed that "a diplomatic path to resolve this issue" was still possible.
The West is accusing Moscow of leading a secessionist revolt in eastern Ukraine after it annexed Crimea last month. Russia strongly denies the claim.
Russia said on Saturday it would "take all possible steps" to secure the release of eight observers from the Organisation for Security and Co-operation in Europe (OSCE) who were seized and accused of espionage by pro-Russia gunmen.
The group - believed to be military observers from Germany, Denmark, Poland, Sweden and the Czech Republic - is being held along with several Ukrainian army personnel in the city of Sloviansk.
Those holding the observers have said they could be exchanged for prisoners held by Ukrainian forces. However, Kiev says they are being used as human shields.
The US state department said Mr Kerry "expressed continued concern that Russia's provocative troop movements on Ukraine's border, its support for separatists and its inflammatory rhetoric are undermining stability, security and unity in Ukraine".
Mr Kerry also "urged Russian support without preconditions for the efforts of the OSCE and the government of Ukraine to liberate the... inspectors and their Ukrainian guides," the statement added.
For his part, Russian Foreign Minister Sergei Lavrov said Ukraine must end military operations in the east of the country as part of urgent measures to defuse the crisis.
Ukrainian troops have carried out a number of raids to try to regain control of official buildings.
The crisis began when protesters toppled pro-Russian President Viktor Yanukovych in February and has since plunged East-West relations to their lowest point since the Cold War.
Russia has tens of thousands of troops deployed along its side of the border with Ukraine and has said it will act if its interests are threatened.
On Saturday, the G7 praised Ukraine for acting with restraint in dealing with the "armed bands" that had occupied government buildings.
But the group, which comprises the US, UK, Germany, Japan, France, Canada and Italy, condemned Russia's "increasingly concerning rhetoric and ongoing threatening military manoeuvres."
The G7 said it was committed to intensifying sanctions on Russia, ahead of Ukrainian presidential elections next month.
The US and EU already have asset freezes and travel bans in place targeting a number of Russian individuals and firms accused of playing a part in the annexation of Crimea.