Russian security forces have detained the suspected killer of a Moscow resident whose stabbing sparked some of Russia's worst anti-migrant riots in years, a spokeswoman for investigators said on Tuesday.

"He has been detained," the spokeswoman for Russia's Investigative Committee told Agence France Presse. Reports said he was detained near Moscow.

Russian investigators said on Tuesday they had detained two suspected Islamists who planned to blow up a chemical storage facility in central Russia.

The two natives of Russia's volatile North Caucasus region had since September been preparing a "terrorist act" at the chemical storage facility in the central Kirov region that could have put the lives of "hundreds of people" at risk, investigators said in a statement.

The Russian agency involved in studying the remains of Yasser Arafat on Tuesday denied issuing any conclusions about the death of the late Palestinian leader, after a report cited its chief as saying he could not have died from polonium poisoning.

Russia's Federal Medical-Biological Agency (FMBA) was one of several international agencies involved in exhuming Arafat's remains in November 2012.

The National Security Agency is gathering email and instant messenger contact lists from hundreds of millions of ordinary citizens worldwide, many of them Americans, The Washington Post reported late Monday.

The U.S. agency's data collection program harvests the data from address books and "buddy lists", the newspaper said, citing senior intelligence officials and top secret documents provided by the fugitive NSA contractor Edward Snowden.

Russia on Monday urged the United States to do everything in its power to bring the Syrian opposition to peace talks after a key group said it would not attend a proposed conference in Geneva.

"We very much expect our American partners and other countries, which not only have influence on various opposition groups but also ... encourage these opposition groups to continue fighting, to realize their responsibility for creating conditions for performing their share of the work for convening Geneva 2," Russian Foreign Minister Sergei Lavrov said.

The Moscow authorities on Monday scrambled to calm inter-communal tensions after the Russian capital was rocked by some of its worst ethnically-fueled rioting in years, sparked by the killing of a Russian allegedly by a Muslim migrant from the Caucasus.

Seeking to avert further protests, Moscow police detained 1,200 people in a raid on migrants as officials vowed to crack down on vegetable wholesale markets in the capital which employ hundreds illegally.

Thousands rioted Sunday in Moscow, bashing in the doors and windows of a shopping center and beating up security guards in a nationalist protest sparked by a murder blamed on a migrant.

The crowd, chanting "Russia for Russians", also beat down the doors of a nearby vegetable warehouse where many immigrants were working, an Agence France Presse photographer reported.

A Russian photographer has been abducted by a group of Syrian rebels who accuse him of being a spy, the Russian foreign ministry said Saturday.

Konstantin Zhuravlev is being held by the Islamist group Liwa al-Tawhid, said foreign ministry spokesman Alexander Lukashevich, adding that he was seized in Syria's second city Aleppo.

U.S. intelligence leaker Edward Snowden warned of dangers to democracy in the first video released of the fugitive since Russia granted him temporary asylum in August.

"If we can't understand the policies and programs of our government we can't grant our consent in regulating them," Snowden said in one of the short video clips posted on the WikiLeaks website Friday night.

Russian Finance Minister Anton Siluanov called on the U.S. on Friday to quickly resolve the political stalemate over the U.S. budget and debt ceiling.

Speaking in a meeting of the Group of 20 finance ministers, Siluanov said: "I would like to express our wish that our U.S. counterparts will find a solution to this as quickly as possible... Otherwise we can't move ahead."