Russia’s security service on Wednesday said it had thwarted "terrorist attacks" in Crimea by Ukrainian military intelligence and beaten back an armed assault by Kiev’s forces.

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

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 was killed in clashes with "sabotage-terrorist" groups sent by the Ukrainian defense ministry on August 8.

An advisor to the head of Ukraine's security agency Yuriy Tandit denied the allegations, telling Interfax-Ukraine news agency that Kiev had no intention of taking back the territory "by force".

The Organization for Security and Cooperation in Europe (OSCE) monitoring the frontier between mainland Ukraine and Crimea -- which Russia annexed in 2014 -- did not report the incident.

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

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

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

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

"On the night of August 8 2016 special operations forces from the Ukrainian defense 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 neighboring state and armored vehicles" but was beaten back by the Russian authorities, the statement said.

Russia seized the Black Sea region from Ukraine in March 2014 following a national referendum that followed the ouster of Kremlin-backed president Viktor Yanukovych by western-backed protests in Kiev.

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

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