Russia said Thursday that it had detained three Ukrainian spies, including one who had allegedly been planning an attack with explosives, as tensions spike around Ukraine's conflict with pro-Russia separatists.

The FSB domestic intelligence agency did not say where or when it had detained the Ukrainians.

It said two of the alleged spies -- a father and son -- were agents of Ukraine's SBU security service who had "travelled to Russia to collect information and take photographs and videos of strategically important enterprises and objects of transport infrastructure."

The pair confessed to having been recruited by the SBU, the FSB said, which offered them a fee of $10,000.

The third Ukrainian, who planned to carry out an attack, "was recruited and acted on the instructions" of a top Ukrainian intelligence officer, the FSB said.

"The attack was planned to be carried out by detonating two improvised explosive devices with a total mass of 1.5 kilogrammes in a TNT equivalent," the FSB said in a statement.

He "was detained red-handed with weapons of destruction on his way to the scene of the crime," the agency added.

It did not specify where or when the attack was to take place.

Separately, the FSB said that last month a high-ranking Ukrainian officer with the defence ministry was found guilty of "preparing crimes against the security of the Russian Federation" and sentenced to six and a half years behind bars.

Sabotage group

It said that he was detained in July for allegedly coordinating and supervising a "sabotage group" in 2016 on the Crimean peninsula, which Moscow seized from Ukraine two years earlier.

The group "planned to blow up the tower of the radio and television transmission centre of Crimea, a mobile gas turbine power plant, a fuels and lubricants warehouse and the mast of the radio centre of the Black Sea Fleet of the Russian Navy," the FSB said. 

The announcements come as tensions around Ukraine's long-simmering conflict with pro-Moscow rebels in two of its eastern regions bordering Russia have escalated in recent weeks, with Kiev's Western allies claiming Moscow is planning to invade Ukraine this winter.

Russia denies the allegations and says Ukraine has deployed half of all its forces near the conflict zone.

The pro-Russian separatists seized the two regions in 2014 shortly after Moscow annexed Crimea.

Kiev and its Western allies say Russia has sent troops and military equipment across the border to bolster the separatists -- claims Moscow denies.

Spy cases have increased in Russia in recent years, especially after ties with the West deteriorated in the wake of the Crimea annexation. 

Many of the cases concern Ukrainians, but some are also related to EU member Poland and the Baltic states.