A Russian court on Thursday arrested a dual Russian-Lithuanian national on treason charges, in a classified case whose details remain unknown.

"Yevgeny Mataitis has been arrested today by the Lefortovo District Court" on charges of state treason, court spokeswoman Yulia Skotnikova told AFP.

NATO head Jens Stoltenberg warned Thursday there was still a risk of heavy fighting in Ukraine and urged Russia to halt its support for pro-Moscow rebels.

In recent weeks there has been an upsurge in fighting in a conflict which has already killed some 6,500 people, despite a February ceasefire brokered in Minsk which has been regularly breached.

Russian President Vladimir Putin on Wednesday extended a ban against most Western food imports for a year after EU foreign ministers agreed to prolong sanctions against Moscow over the Ukraine conflict until January 2016.

"The government turned to me with an appeal to extend the measures," Putin told a government meeting.

Islamist militants in four regions of Russia's Caucasus have pledged allegiance to the Islamic State group, according to a recording which was welcomed by IS after being posted online.

The voice recording posted on YouTube Sunday said militants in Russia's Dagestan, Chechnya, Ingushetia and Kabardino-Balkaria regions had all swore fealty to Abu Bakr al-Baghdadi, head of the feared jihadist group.

Japanese Prime Minister Shinzo Abe called on Russian President Vladimir Putin Wednesday to play a constructive role in finding "peaceful and diplomatic solutions" to the crisis in Ukraine, after the EU agreed to prolong sanctions against Russia.

During a 30-minute evening teleconference, Abe requested that Putin implement fully the Ukraine ceasefire deal agreed in February, Japan's foreign affairs ministry said in a statement.

The mysterious Russian agency that hires people to write pro-Kremlin propaganda on the web stepped into the public spotlight for the first time Tuesday as a former employee took it to court.

The Agency for Internet Studies, which runs what has been called a "troll factory" from a nondescript Saint Petersburg address, is being sued by employee Lyudmila Savchuk for alleged underpayment and various labor violations.

The U.S. will pre-position heavy weapons, including tanks, infantry fighting vehicles and artillery in central and eastern Europe for the first time, U.S. Defense Secretary Ashton Carter said Tuesday in Estonia's capital Tallinn.

"We will temporarily stage one armored brigade combat team's vehicles and associated equipment in countries in central and eastern Europe," Carter said at a joint press conference with three Baltic defense ministers.

Ukrainian rebels Tuesday accused government forces of killing three civilians in attacks launched hours before the start of talks in Paris on ways to halt the 15-month separatist war.

Kiev's Western-backed military command reported the death of one soldier and accused the pro-Russian rebels of launching the heaviest wave of mortar and artillery fire since the start of the year in one relatively peaceful region.

Iran's parliament approved a controversial bill Tuesday that it said would protect the country's nuclear program, but which could also hinder final negotiations on a deal with world powers.

The move re-exposed persistent tension between President Hassan Rouhani's government and lawmakers in Tehran, where hardliners routinely voice doubt about the merit of talking to the West.

Former Ukraine president Viktor Yanukovych has thanked Russian President Vladimir Putin for "saving my life" during the bloody demonstrations that led to his ouster.

Yanukovych, currently exiled in Russia, told the BBC that he would like to return to Ukraine one day and blamed the country's plight on the demonstrators who occupied Kiev's Maidan Square -- the focal point of the uprising against his rule.