“This event over the next few days is Exhibit A of how we’re working with international partners to disrupt ransomware networks,” the official said.

Participating nations include longtime U.S. allies such as the U.K., Australia, Germany and Israel, as well as Bulgaria, Kenya, Nigeria and the United Arab Emirates. The list underscores “just how pernicious and transnational and global the ransomware threat has been in the different countries from all different parts of the world who will be participating,” the official said.

The U.S. official didn’t give specific reasons for why the Russian government wasn’t invited to this week’s summit but said the country’s absence at this meeting didn’t preclude its involvement in future such meetings. The U.S. has blamed Russian hackers for launching cyberattacks, including a campaign against technology provider

SolarWinds Corp.

that resulted in corrupted software being sent to nine U.S. agencies and several companies. Moscow has consistently denied involvement in cybercrimes or that it harbors cybercriminals.

The U.S. is continuing to discuss the issue of ransomware with Russia through bilateral channels instead, the official said, adding that Russia has acted on information shared through those discussions to disrupt ransomware operators.

“Clearly, Russia plays a role because of a number of criminal actors who are operating from Russia. And that is the reason that President Biden established, with President Putin, a dedicated channel for us to have very focused and candid discussions,” the official said.

Homeland Security Secretary Alejandro Mayorkas said in September that ransomware attacks in the U.S. have increased by 300% in the past year.

After high-profile attacks on Colonial Pipeline Co. in May, which disrupted East Coast fuel supplies, and meatpacker

JBS SA

in June, the U.S. government has dedicated significant resources to cybersecurity efforts.

These include a Justice Department task force focused on ransomware, an executive order from President Biden in May and sanctions in September for a Russia-based cryptocurrency exchange the Treasury Department accused of managing payments for cybercriminals.

Cybersecurity experts have long warned that serious efforts to mitigate the impact of ransomware attacks, which disrupted operations at hospitals and other critical infrastructure during the coronavirus pandemic, are unlikely to succeed without Russian involvement.

“I’ve been in cybersecurity for 24 years and I’ve never seen it this bad,” said

Tom Kellermann,

head of cybersecurity strategy at software company

VMware Inc.

“I’ve never seen cybercrime cartels this emboldened, this organized, this sophisticated and so punitive in their actions.”

—Catherine Lucey contributed to this article.

Write to James Rundle at james.rundle@wsj.com