Putin warns of Ukraine civil war as Kiev moves on rebels

Russian President Vladimir Putin has warned that Ukraine is "on the verge of civil war" after Kiev sent in troops against pro-Russia separatists.

Putin, in a phone call to German Chancellor Angela Merkel, described the move as a "sharp escalation".

Ukraine announced the start of an "anti-terrorist operation" in the east of the country on Tuesday.

Moscow annexed Crimea last month and is believed to have thousands of troops massed along its border with Ukraine.

"The Russian president remarked that the sharp escalation of the conflict has placed the country, in effect, on the verge of civil war," the Kremlin said of Putin's conversation with Merkel.

But it said that both leaders "emphasised the importance" of planned four-way talks on Thursday between senior diplomats from Russia, the EU, the US and Ukraine.

The Kremlin statement described the Ukrainian military operation in eastern Ukraine as an "anti-constitutional course to use force against peaceful protest actions".

Tensions have mounted in recent days after pro-Russian rebels seized buildings in about 10 towns and cities across eastern Ukraine.

They are demanding greater autonomy or referendums on secession from Ukraine.

UK Foreign Secretary William Hague said Russia had sent in "thinly disguised" armed groups to spearhead the occupation of buildings.

Hague underlined that Moscow faced "serious long-term consequences" if it continued to destabilise Ukraine.

Kiev and Washington have accused Russia of fomenting the unrest, something Moscow strongly denies.

The White House has described Ukraine's military operation as a "measured" response to a lawless insurgency.

Russia took control of the Ukrainian region of Crimea last month, after it held a controversial referendum on self-determination.

The US has said it is "seriously considering" adding to sanctions imposed after the annexation of Crimea.