Russia's security service on Wednesday said it had thwarted "terrorist attacks" in Crimea by Ukrainian military intelligence and beaten back an armed assault, but Kiev denied the claims.

The FSB said in a statement that one of its officers was killed in armed clashes while arresting "terrorists" on the night of August 6-7 while a Russian soldier died in a firefight with "sabotage-terrorist" groups sent by the Ukrainian defence ministry on August 8.

The allegations will fuel further tensions in the feud between Russia and Ukraine, sparked when Moscow annexed Crimea from Kiev in March 2014 after Kremlin-backed president Viktor Yanukovych was ousted.

Ukraine's national security council chief Oleksandr Turchynov blasted the claims as "hysterical and false" and said Moscow was trying to stoke fear in Crimea.

Ukraine's defence ministry dismissed the allegations as "nothing more than an attempt to justify the redeployment and aggressive actions" of Russian forces in the region.

The Organisation for Security and Cooperation in Europe (OSCE) monitoring the frontier between mainland Ukraine and Crimea did not report any incidents.

But it said road traffic was halted this earlier this week and border guards appeared to be on "heightened alert".

In its statement, the FSB -- which controls Russia's border guards -- said it had "foiled terrorist attacks on the territory of Crimea prepared by the intelligence directorate of the Ukrainian defence ministry".

"The aim of the sabotage and terrorist attacks was to destabilise the social and political situation" ahead of elections in Russia and Crimea next month, it said.

The security agency said that in the August 6-7 raids, several people were detained, including a Ukrainian military intelligence officer, and a cache of explosives was discovered.

"On the night of August 8 2016 special operations forces from the Ukrainian defence ministry carried out two more attempts to make a breakthrough by sabotage-terrorist groups," it said.

The assault included "massive firing from the side of the neighbouring state and armoured vehicles" but was beaten back by the Russian authorities, the statement said.

The FSB said it had stepped up security measures around the peninsula following the alleged incidents.

- Election tensions -

Russia seized Crimea in 2014 after sending in thousands of special forces troops to take control of Ukrainian bases and holding a hastily-organised referendum that was rejected by the international community.

The move shattered ties between the two ex-Soviet neighbours and sent relations between Moscow and the West plummeting to their lowest point since the Cold War.

Since its annexation by Russia, Crimea has remained largely peaceful.

However, a separatist conflict -- that the West and Kiev blame on Moscow -- has killed some 9,500 people in two regions of Ukraine's industrial east.

Fighting in the east between Ukrainian forces and pro-Russian rebels is still rumbling on as a peace deal to end the violence fails to make progress.

Russia is gearing up for nationwide legislative elections on September 18 and President Vladimir Putin has warned his security officials that "foreign enemies" are seeking to disrupt the vote.

Crimea governor Sergei Aksyonov said any attempts to destabilise Crimea at the height of the tourist season will be prevented "in the harshest possible way."

"The safety of residents and guests of the peninsula is fully assured," he wrote on Facebook.

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