June 15 (Reuters) - Cybersecurity leaders at major U.S. firms, including Nvidia ( NVDA ) and Adobe, have asked the Trump administration to lift restrictions on Anthropic's most powerful AI models, arguing that the bans hamper efforts to prevent the spread of digital attacks.

The letter follows Washington's decision on Friday ordering Anthropic to suspend access to its Fable 5 and Mythos 5 models for any foreign nationals over national security concerns.

After previously warning about the hacking capabilities of its Mythos model and withholding it from wide release to prevent potential harm, Anthropic last week released a public version called Fable with what it described as cybersecurity safeguards.

The curbs that Washington has now placed on the technology will limit the cybersecurity industry's ability to find and fix software flaws at a time when other AI tools are making it easier for hackers to exploit vulnerabilities, according to a letter on Sunday signed by more than 50 security leaders.

The letter said Anthropic's models were not uniquely capable of finding security flaws and weaponizing exploits, with rival models, including China's Kimi 2.7, offering similar abilities.

"Mythos is almost definitely the best model right now for finding security bugs and codes, but it is like an incremental advance over other models that are already open," Joshua Saxe, CTO of AI security firm Abundant Security and a signatory of the letter, said in an interview.

ANTHROPIC'S NATIONAL SECURITY TIGHTROPE

Senior Anthropic staff are scheduled to meet with government officials at the U.S. Department of Commerce in Washington on Monday, an official in the Trump administration told Reuters.

Anthropic has said the government believes there is a way to bypass, or "jailbreak," a safeguard that prevents Fable from being used to identify software vulnerabilities. It has argued that a narrow potential jailbreak should not be grounds for cutting off access to a model used by hundreds of millions of people.

The letter echoed the point, saying Anthropic has already built robust protections and that pulling the capabilities could prove "dangerous" as China's open-source models are just months behind the best American ones, with Beijing likely having access to capabilities beyond what is publicly known.

Any regulation needs to be evidence-based, clearly defined and applied consistently and "none of those standards was followed here," said Alex Stamos, another signatory who serves as chief product officer at Corridor.

"This is an overreaction by the government," he said, adding that there was a dispute between Anthropic and the third party that flagged the issue over how serious the findings were, based on his conversations with those involved.

Cybersecurity company CrowdStrike last week said China-linked hackers posed the biggest espionage threat to technology companies over the past year.

The $965 billion AI company, which is preparing to go public, has previously tussled with the U.S.government on access to its models and their impact on national security.

The Trump administration earlier this year directed U.S. agencies to stop working with Anthropic and declared it a supply risk due to its reluctance to let its technology be used for mass surveillance and autonomous weapons.

(Reporting by Zaheer Kachwala in Bengaluru; Editing by Arun Koyyur and Anil D'Silva)