MOSCOW (AFP) - Russia said on Thursday (Dec 2) 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 Ukrainian SBU, the Russian FSB said, which offered them a fee of US$10,000 (S$13,663).

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 1.5kg in a TNT equivalent," the Russian 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 Russian 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.

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 Russian FSB said.

The announcements come as tensions around Ukraine's long-simmering conflict with pro-Moscow rebels in two of its eastern regions...