Jaume Duch,

a spokesman for the parliament, tweeted that the legislative body’s websites were down because of denial-of-service, or DDoS, attacks. Parliament teams are working to fix the problem, he said. 

The European Parliament website was back online Wednesday evening Brussels time after a few hours offline.

The Pro-Russia hacker group Killnet was behind the attack, a Parliament official said, adding that confidential material hadn’t been penetrated. DDoS attacks are typically unsophisticated, temporarily making a website unavailable by overloading it with internet traffic. 

“Our IT experts are pushing back against it & protecting our systems,” Ms. Metsola tweeted. “My response: #SlavaUkraini,” she wrote, referring to a phrase that means “Glory to Ukraine.”

A computer emergency response team based in Brussels is responsible for responding to cyberattacks on EU institutions including the parliament. The office didn’t respond to a request for comment.

The attack didn’t appear to have serious effects, said Dr.

Lukasz Olejnik,

an independent cybersecurity researcher and consultant, and formerly a cyberwarfare adviser at the International Committee of the Red Cross. Killnet typically attacks organizations because it wants its hacks to receive attention and cause discussion among policy makers, he said. “In other words, it’s a type of information operation,” he said.

Killnet has targeted companies and government agencies in Ukraine and countries that support Ukraine, including Italy and the U.S. The group briefly took down New York’s LaGuardia, Los Angeles International and several other American airports’ websites in October. 

The European Parliament’s resolution Wednesday called Russia a state sponsor of terrorism and said the country’s targeting of Ukrainian civilians violates international law. The resolution isn’t legally binding.

Hacktivists that support Russia, and others that support Ukraine, have targeted companies during the war. After the attack on U.S. airports last month, cloud and cybersecurity services company

Cloudflare Inc.

said it appeared that Killnet was increasingly targeting organizations outside Ukraine. DDoS attacks in the third quarter of this year increased by 111% compared with the same period last year, Cloudflare said.

—Laurence Norman contributed to this article.

Write to Catherine Stupp at catherine.stupp@wsj.com