Ukraine's security service (SBU) said it has foiled a Russian plot to kidnap and assassinate President Volodymyr Zelenskyy and kill other top Ukrainian officials.

Two serving colonels from the Ukrainian government's own protection unit have been arrested in a troubling sign of Russian penetration of Ukraine's security apparatus. The SBU said the men were part of what it called a "network of agents" that had been working for the Russian security services.

Other intended targets in the conspiracy included Vasyl Malyuk, the head of the SBU, and Lieutenant General Kyrylo Budanov, the chief of Ukraine's military intelligence agency. The plotters had intended to kill the military spy chief before the Orthodox Easter, which this year fell on 5 May, according to a statement by the SBU.

Mr Malyuk said the plot was supposed to be a "gift to Putin before the inauguration" but was "actually a failure of the Russian secret service". "But we must not forget that the enemy is strong and experienced, and cannot be underestimated.

We will continue to work proactively to ensure that every traitor receives a well-deserved court sentence," he said. Russia has previously been accused of multiple plots to kill Mr Zelenskyy since President Vladimir Putin launched his full-scale invasion of Ukraine in February 2022.

But the latest scheme is particularly notable because of the alleged involvement of two colonels in the State Guard of Ukraine, which is tasked with protecting top officials. Read more on Sky News:Ukrainian mother captured by Russia describes ordealWhy won't Germany provide Ukraine with better weapons? Russia's intelligence services recruited the men before the full-scale war, the Ukrainian security service said.

It claimed that the Russian agents had been trying to find members of the Ukrainian military close to President Zelenskyy's security detail who would be willing to take him hostage and kill him. A broader plan involved identifying the location of a senior Ukrainian official and targeting the site with a missile strike, followed by a drone attack and then a second missile strike.

The SBU said the whole operation was run from Moscow and it published the names of three Russian intelligence officers who were allegedly behind the plot. The two Ukrainian colonels were arrested on suspicion of treason, which carries a life sentence.

The head of the SBU said he "personally controlled" the investigation, which "gradually" built up evidence against the two colonels. The SBU said the suspects were part of a network working for the FSB, Russia's security service, and had also leaked classified information..