Mattis said the attempted attack on the Organisation for Prohibition of Chemical Weapons (OPCW) showed how cyber attacks were becoming "more frequent, more complex and... more destructive".

"This is why the United States, like the United Kingdom, Denmark, the Netherlands, Estonia will provide national cyber contributions to help NATO fight in this important domain," Mattis told reporters at a meeting of NATO defence ministers.

The Netherlands revealed extraordinary details of the plot by Russia's GRU military intelligence agency against the OPCW in The Hague, including photos of the alleged agents and their equipment.

Mattis refused to be drawn on what kind of response the OPCW plot should be met with, saying "tit for tat on cyber" was not necessarily appropriate, but he insisted the Kremlin must face consequences.

"Basically the Russians got caught with their equipment, with their people who were doing it and they have got to pay the piper. They are are going to have to be held to account," Mattis said.

"We have a wide variety among our nations of responses available to us."

At the time of the attack, the OPCW was investigating the nerve agent poisoning of former Russian spy Sergei Skripal and his daughter Yulia in Salisbury, England. Dutch officials said it was not clear if the cyber operation was linked to that.

Mattis said the US would offer cyber support to NATO allies immediately, but refused to go into detail about the kind of capabilities it would make available.

NATO says its forces have successfully used offensive cyber techniques to disrupt Islamic State and Al-Qaeda operations in Iraq.

The alliance has recognised cyber as a full conflict domain alongside air, land, sea and space and said that a cyber attack could trigger its Article 5 mutual defence pact.

NATO's Baltic members -- Lithuania, Latvia and Estonia -- say they come under near-daily cyber assault, with government departments, banking systems and the power grid coming in for attack, and point the finger at former Soviet ruler Russia.