Pro-Russian hackers have been blamed for forcing the websites of Dutch commercial ports offline last week.

Groningen Zeehaven’s site was down all weekend, while Amsterdam, Rotterdam and Den Helder were all offline for several hours on Tuesday.

The sites were targeted by DDoS attacks, in which their servers are bombarded with simultaneous access requests until they exceed their capacity.

Systems that handle shipping traffic, which run on different servers, were not affected.

A group of pro-Russian hackers known as NoName0157(16) said it carried out the attacks in response to the Netherlands’ decision to supply Leopard tanks to Ukraine, RTL Nieuws reported.

Tom Hegel, of US-based cybersecurity company SentinelOne, said the group was a small collective of hackers who specialised in taking down western websites.

“They use amateurish tools but they’re effective,” he told RTL. “It’s good enough to achieve their aim of taking websites down and getting attention. And then they boast about their successes on their Telegram channels.”

The port authorities said the attacks were disruptive but did not cause serious damage. “We had a big open day last Saturday, so it wasn’t very convenient,” a spokesman for Groningse Zeehaven said.

The Port of Rotterdam said it had identified Russia as the source of the attacks, with most of the access requests coming from Russian and Serbian internet addresses.

“Our website is important to us because it lets us inform the public, but we’re not dependent on the website,” a spokesman said.