Dutch intelligence services have provided "crucial evidence" to US counterparts about Russian meddling in the 2016 elections, Dutch media reported Friday.

After penetrating the network of Russian hackers known as Cozy Bear, the Dutch agents alerted US spy chiefs as they watched the Russians transfer "thousands" of Democratic Party emails ahead of the presidential vote.

The Dutch national intelligence service (AIVD) had been watching the notorious group since 2014, according to the respected Volkskrant daily, and a Dutch TV news programme Nieuwsuur.

They have provided "crucial evidence of the Russian involvement" as well as "technical" proof in the hacking of the US Democratic Party, the reports said, citing anonymous US and Dutch sources.

After digging themselves by chance into the Russians' computer network run out of a university building near Moscow's Red Square, the Dutch agents also gained access to the security cameras surveilling the room.

"Not only can the intelligence service now see what the Russians are doing, they can also see who's doing it," the Volkskrant said.

The AIVD alerted its US counterparts when in 2015 it became a "witness to the Russian hackers harassing and penetrating the leaders of the Democratic Party, transferring thousands of emails and documents," it said.

"Yet, it will be months before the United States realise what this warning means: that with these hacks the Russians have interfered with the American elections. And the AIVD hackers have seen it happening before their very eyes."

Pictures obtained from the security cameras, led the AIVD to conclude that Cozy Bear is directed by Russia's external intelligence agency, the SVR.

Cozy Bear has been widely blamed for meddling in the American elections won by Republican President Donald Trump.

Despite Trump's vehement denials of working with the Russians, his White House is now under investigation for possible collusion by special prosecutor Robert Mueller.

The information provided by the Dutch was "relevant material" to Mueller's team, Volkskrant said.

In the past eight months, members of Trump's campaign, family and White House staff have all been interviewed, and four indictments have been issued, with two guilty pleas as the investigation has spiralled closer to Trump himself.

A spokeswoman for the AIVD refused to confirm the Dutch media reports, telling AFP "we never comment on operations."

Kremlin spokesman Dmitry Peskov however dismissed the reports Friday, saying Moscow had "never heard or seen anything about such allegations from the Dutch secret services".

"If Dutch newspapers want to throw oil on the fire of the anti-Russian hysteria in the United States, that's not the most dignified of activities," Peskov told reporters.

He added it was "not the best thing" to believe what was in Dutch newspapers.

- 'Information goldmine' -

But American intelligence agencies have said "with high confidence" they believe the Kremlin was behind the attack on the Democratic Party.

That confidence is based on "AIVD hackers having had access to the office-like space in the centre of Moscow for years," the Volkskrant said.

Earlier in November 2014, the Dutch also alerted the US to a Cozy Bear attack on the State Department, the daily added.

Even though the assault was thwarted after a 24-hour cyber battle, one Trojan email did get through, enabling the Russian group to access the White House.

The Dutch infiltration proved to be "a goldmine," Volkskrant said. It led to high level meetings between the country's top spy chiefs at the time, while the Americans also handed over knowledge, technology and intelligence.

The spying on Cozy Bear lasted between a year to two and a half years, the Dutch media said, adding the AIVD hackers were no longer in the group's network.

*Updated with Russian reaction

Related: U.S. Vows Response to Russian Hack at 'Time and Place of our Choosing'

Related: Hacking of DNC Raises Fears of Cyber Attack on U.S. Election

Related: US Election - Official Probe Slams Clinton's Private Email Use