* Head of GRU military intelligence says situation 'tense'

* Says NATO members, Ukraine, Israel arming Tbilisi

(Recasts, adds quotes, details)

By Conor Humphries

MOSCOW, Nov 5 (Reuters) - Russian military intelligence believes Georgia might again attack South Ossetia, the pro-Moscow region over which the two countries fought a war last year, a powerful spy chief said on Thursday.

Alexander Shlyakhturov, who in April took over command of the military's Main Intelligence Directorate (GRU), said the situation was strained and accused NATO countries of continuing to supply arms to Georgia.

"The situation with Georgia remains tense because the current Georgian authorities do not just refuse to recognise the sovereignty of Abkhazia and South Ossetia, but are trying in every way to return these countries...to their jurisdiction," he said in a rare interview with state news agency ITAR-TASS.

"You have to add to this the unpredictability of attempts by the Georgian leadership, headed by (President Mikheil) Saakashvili, which may give in to temptation to use force to tame these obstinate republics as they did last year," he said.

"We do not rule out such a development," said Shlyakhturov, who controls Russia's biggest spy agency with agents across the globe and thousands of special forces troops inside Russia.

South Ossetia and Abkhazia broke away from Georgian government control in the early 1990s. Russia recognised both as independent states after last year's five-day war, when its forces repelled a Georgian attack on South Ossetia.

But only two other countries, Nicaragua and Venezuela, have followed Russia's lead, and the rest of the world regards the two regions as part of Georgia. Saakashvili has said he is committed to a unified Georgia but has no pretensions to retake the breakaway areas by force.

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GRU chiefs rarely speak to the media. Thursday's interview was given to mark 91 years since the spy service was created in 1918 by revolutionary leader Leon Trotsky.

Shlyakhturov told TASS that "new NATO members" in Eastern Europe were supplying small arms and munitions to Georgia while Israel was providing aerial drones.

He said heavy artillery and anti-aircraft systems were being delivered to Georgia from Ukraine. GRU forces fought on the front line of last year's war against Georgia.

"Georgia is reviving its military potential with the supplies of arms and military equipment from foreign countries," he said. "They could well start another such adventure."

South Ossetian authorities on Thursday said they had detained four armed Georgian teenagers in the breakaway capital, Tskhinvali, for illegally entering South Ossetia.

"Four grenades and other explosive materials were seized from the teenagers," South Ossetia's government said in a statement. The Georgian Interior Ministry called for their immediate and unconditional release. (Editing by Mark Trevelyan)

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