Photo from UNIAN

An undercover operation by the SBU security service documented a Russia-sponsored plot, one in a series, to further sow discord in Ukrainian-Polish relations.

The SBU Security Service of Ukraine in Lviv region says earlier reports about a burned Polish bus in Lviv was not an April Fool's joke, adding that "terrorists have been detained."

SBU press secretary in Lviv region Oleh Oleksandrov said that the attackers had been under constant surveillance from the very start, to ensure that any casualties are avoided.

"I must clear up the reports that have stirred the media. It's about a Polish bus set on fire on April 1 near the Stryi bus station in Lviv. This incident took place within the framework of an undercover operation to document a crime. Security operatives had actions of terrorist under control to avoid casualties and document the moves of the entire team of perpetrators – from direct executors to those who ordered the act. The attackers were detained April 1. The next day, on April 2, they were charged, and on April 3, the court ruled to remand them in custody pending trial," Oleksandrov said.

Read alsoRussia's covert campaign underway to inflame East Europe - mediaAs UNIAN reported, SBU Chief Vasyl Hrytsak said on Tuesday that the special services had detained two executors of the blast that rocked a Polish memorial at the Lychakiv cemetery in Lviv, who also set fire to a Polish tourist bus.

"According to the information available, terrorist attacks were financed by allies of a former Ukrainian president [Viktor Yanukovych] under the supervision of Russian intelligence," he said.

On March 13, a blast occurred near a Polish memorial at a Lychakiv cemetery in Lviv. According to preliminary police reports, an RGD-5 grenade exploded, resulting in a 20cm-wide funnel forming. Nobody was hurt in the incident. The memorial was not damaged either.

Read alsoPoland's security forces bust group involved in arson attack on Hungarian culture center in ZakarpattiaOn April 1, the media and people on social networks reported that unknown perpetrators had allegedly set fire to a Polish bus near the Stryi bus station in Lviv at night, also pasting it with red-and-black leaflets. The initial reports were ruled out by local officials as an April Fool's joke.