By Alexei Anishchuk

SOCHI, Russia (Reuters) - Russian President Vladimir Putin warned ex-Soviet allies on Monday that Islamist militancy fuelling the war in Syria could reach their countries, some of which have Muslim majorities.

He said Russia and its allies would provide "additional collective assistance" to Tajikistan to guard its border with Afghanistan after the pullout of most foreign combat troops in 2014.

Russia, which has a large Muslim minority of its own and is fighting an Islamist insurgency, has accused the West of helping militants by seeking Syrian President Bashar al-Assad's removal without paying enough attention to the potential consequences.

Putin told leaders of the six-nation Collective Security Treaty Organisation (CSTO) that militants fighting Assad could eventually expand attacks beyond Syria and the Middle East.

"The militant groups (in Syria) did not come out of nowhere, and they will not vanish into thin air," Putin said.

"The problem of terrorism spilling from one country to another is absolutely real and could directly affect the interests of any one of our countries," he said, citing the deadly attack on a shopping mall in Nairobi as an example.

"We are now witnessing a terrible tragedy unfold in Kenya. The militants came from another country, as far as we can judge, and are committing horrendous, bloody crimes," Putin said at a CSTO summit in the Russian Black Sea resort city of Sochi.

His words appeared to be a warning about violence spreading from both Syria and Afghanistan, which shares a long border with CSTO member Tajikistan in Central Asia.

BORDER ASSISTANCE

Reiterating concerns violence could spread to former Soviet Central Asia and Russia after the pullout of most foreign combat troops from Afghanistan by the end of 2014, he said CSTO nations agreed to draft a plan to protect the border.

"We will provide additional collective assistance to Tajikistan to strengthen the Tajik-Afghan state border," Putin said. He gave no details.

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Russian border guards used to patrol the Tajik frontier with Afghanistan but left in 2005.

The CSTO security alliance also includes Kazakhstan, Kyrgyzstan, Armenia and Belarus. Tajikistan, Kazakhstan and Kyrgyzstan all have mostly Muslim populations. Central Asian states Uzbekistan and Turkmenistan, neither members of CSTO, also have frontiers with Afghanistan.

Russian officials have expressed concern that Russian-born militants fighting in Syria could return to Russia's North Caucasus and join an insurgency that claims lives almost daily.

Russia has been one of Syria's strongest backers in a conflict that has killed more than 100,000 people since it began in March 2011, delivering arms to Assad's forces and joining China in blocking Western-backed initiatives in the U.N. Security Council.

Russia, which has echoed Assad's contention that he is fighting al Qaeda-inspired Islamists rather than a popular revolt, has warned the West that military intervention in Syria would play into the hands of the militants.

Backing Moscow, the CSTO nations issued a joint statement after the summit saying that military action or other foreign intervention in Syria was "unacceptable" and would be illegal if carried out without the approval of the U.N. Security Council.

(Editing by Alistair Lyon)