The planned attacks were allegedly aimed at police forces and an Orthodox Christian church.

FSB said it had arrested three Russian citizens in the city of Kaspiysk and identified them as members of an international terrorist organisation, but did not name it.

It said investigators had seized homemade explosive devices and other weapons.

Islamist violence has flared up again in recent months in the North Caucasus, which in the 1990s and 2000s was riven by wars and insurgencies emanating from Chechnya.

Last month, 22 people were killed in simultaneous attacks on churches, synagogues and police checkpoints in two cities.

In March, an attack claimed by Islamic State killed 145 people at a concert hall on the outskirts of Moscow.

Russia has arrested the suspected gunmen and a number of alleged accomplices, mostly citizens of Tajikistan, who are currently awaiting trial.