The US secretary of state, John Kerry, claims America has obtained intercepted phone calls that prove Moscow is deliberately trying to destabilise eastern Ukraine, according to reports of leaked remarks he made at a private meeting last week.

US news site the Daily Beast quoted Kerry saying: "Intel is producing taped conversations of intelligence operatives taking their orders from Moscow … We know exactly where they are coming from."

The leaked comments came as masked pro-Russian gunmen took control of a government building in another eastern Ukrainian town, north of the separatist stronghold of Donetsk. They were seen controlling official buildings in Horlivka, a day after hundreds of separatists seized a police headquarters in nearby Luhansk.

They wore the same military uniforms without insignia as other so-called "green men" who have joined pro-Russian protesters in seizing control of a string of towns in the eastern Donbass region.

The Daily Beast said it had a recording of Kerry's remarks, which were made during a private meeting last Friday in Washington of the Trilateral Commission, a thinktank that aims to foster co-operation between America, Europe and Japan.

It quoted him claiming that the intercept evidence disproved Russian denials about involvement in the separatist unrest.

Kerry said: "It's not an accident that you have some of the same people identified who were in Crimea and in Georgia and who are now in east Ukraine,

"This is insulting to everybody's intelligence, let alone to our notions about how we ought to be behaving in the 21st century. It's thuggism, it's rogue state-ism. It's the worst order of behaviour."

The state department played down the leak, saying Kerry was referring to Ukrainian rather than US intelligence.

In her daily briefing to journalists, the department's spokeswoman, Jen Psaki, pointed out that Kerry had already made similar claims in public.

At a briefing for reporters last Thursday, He said: "Our intelligence community tells me that Russia's intelligence and military intelligence services and special operators are playing an active role in destabilising eastern Ukraine with personnel, weapons, money, operational planning and coordination. The Ukrainians have intercepted and publicised command-and-control conversations from known Russian agents with their separatist clients in Ukraine."

Kerry did not mention Ukrainian intelligence in his leaked comments the next day, according to the Daily Beast.

It claimed his remarks suggested the US National Security Agency had found a way of eavesdropping on the Russian military.

• This article was amended on 1 May 2014. The earlier version said "deliberating trying to destabilise" where it should have said "deliberately trying to destabilise".