Dutch intelligence thwarted a Russian cyber attack targeting the global chemical weapons watchdog in April and expelled four Russian agents, the government said on Thursday.

The Russians set up a car full of electronic equipment in the car park of a hotel next to the Organisation for the Prohibition for Chemical Weapons in The Hague in a bid to hack into its computer system, it said.

“The Dutch government finds the involvement of these intelligence operatives extremely worrisome,” Dutch Defence Minister Ank Bijleveld told a news conference.

“Normally we don't reveal this type of counter-intelligence operation.”

The Netherlands publicly identified the alleged Russian agents and said the operation was carried out by Russia's GRU military intelligence agency, Dutch officials said.

Britain helped the Netherlands with the operation, they added.

A laptop belonging to one of the four was linked to Brazil, Switzerland and Malaysia.

The activities in Malaysia were related to the investigation into the 2014 shooting down of flight MH17 over Ukraine, Bijleveld added.

At the time of the attack the OPCW was investigating the nerve agent poisoning of former Russian spy Sergei Skripal and his daughter in Salisbury, England.

Dutch officials said it was not clear if the cyber operation was linked to that.

The head of the Dutch MIVD intelligence service, Major-General Onno Eichelsheim, told the news conference that the men travelled to Amsterdam's Schiphol Airport on April 4 on Russian diplomatic passports.

An official from the Russian embassy escorted them to The Hague, he said.

On April 11 they then hired a Citroen C3 and scouted the area around the OPCW — all the time being watched by Dutch intelligence.

“They were trying to commit a close access hack operation,” he said.

'Clearly not on holiday'

The Russians set up in the Marriott Hotel next door to the OPCW and took photos, while parking the car at the hotel with the boot facing the OPCW, he said.

In the boot was electronic equipment to intercept the OPCW's WiFfi as will as log in codes at the organisation, with the antenna hidden in the back of the car facing the OPCW.

“We intercepted it and expelled the four men from the country. It was a successful operation.”

The Dutch spy chief said the Russians had originally taken a taxi from a GRU base in Moscow to the airport, and some of their mobile phones were activated in Moscow near the agency's headquarters.

When leaving The Hague, the men took all the rubbish from their room in a further bid to cover their tracks. “They were clearly not here on holiday,” said Eichelsheim.

News of the Dutch operation came a day after Britain and Australia blamed the GRU for some of the biggest cyber attacks of recent years — including one on the Democratic National Committee during the 2016 US presidential campaign.

They said the Russian military intelligence service could have only been conducting operations of such scale on Kremlin orders.

Russian President Vladimir Putin has repeatedly and angrily rejected similar charges.

He told US President Donald Trump during a July summit in Helsinki that talk of Russia meddling in the 2016 election was “nonsense”.