GENEVA (Reuters) -A pro-Russian hacking group had intensified its cyberattacks against Switzerland, authorities said on Tuesday, with hackers claiming to have taken down several major websites including the one for Geneva Airport.

Switzerland's main government websites, including parliament and the federal administration, have been hit in recent days by a distributed denial-of-service (DDoS) attack claimed by the NoName group.

In a statement on Tuesday, Switzerland's National Cyber Security Centre (NCSC) described the intensity of the DDoS attack as "exceptionally high" and warned some government websites could remain inaccessible.

The attack coincides with preparations by the Swiss parliament for a video address by Ukrainian President Volodymyr Zelenskiy scheduled for Thursday.

"The parliamentary services are doing everything in their power to ensure that the live broadcast on Thursday can go smoothly," NCSC said.

The NoName group said it had carried out an attack against the parliament's website last week in response to Switzerland's adoption of another EU sanctions package against Russia, which has been subjected to Western sanctions following its February 2022 invasion of neighbour Ukraine.

On Tuesday it attacked other websites including that of Geneva Airport, a hub for diplomats and officials travelling to the United Nations.

"The website of the Geneva International Airport did not resist our attack," the group wrote on the Telegram messenger service, posting an image of a bear flaunting its claws.

The airport confirmed its website had been targeted by a DDoS attack since Tuesday morning. It said the attack did not affect its operations.

(Reporting by Gabrielle Tétrault-Farber; Editing by Alexandra Hudson and Grant McCool)