WASHINGTON, March 11 (Reuters) - The Pentagon has told its senior leaders that use of Anthropic's AI tools may continue beyond a previously announced six-month phase-out period if deemed critical to national security, according to an internal memo seen by Reuters.

The memo is dated March 6 and signed by Pentagon Chief Information Officer Kirsten Davies. It says the exemptions can be authorized "in rare and extraordinary circumstances" and "will only be considered for mission-critical activities directly supporting national security operations where no viable alternative exists."

Any Pentagon unit seeking an exemption must submit a comprehensive risk mitigation plan for approval, according to the document, first reported by CBS News.

The Pentagon confirmed the memo but declined to comment further. Anthropic did not immediately respond to a request for comment.

An expert said the carve-out signals how challenging it will be to implement the ban on Anthropic.

The memo is a "recognition of the fact that it's really hard for most vendors to certify they have removed the company from the entirety of their supply chain," said Franklin Turner, a government contracts lawyer at McCarter & English. For instance, contractors may find it difficult to ensure their software is free of any open-source code originating from Anthropic, he said.

    "I do expect to see a flurry of waiver requests," he added.

The memo comes after a heated weeks-long dispute over technology guardrails on use of Anthropic's AI tools by the military that culminated in Defense Secretary Pete Hegseth labeling the firm a supply chain risk and banning its use by the Pentagon and its contractors.

Anthropic filed a lawsuit on Monday to block the Pentagon from implementing the ban.

The memo also directed officials to prioritize removing Anthropic's products from systems supporting critical missions, such as nuclear weapons and ballistic missile defense.

The memo also reaffirmed that the ban extends to defense contractors. It gives Pentagon contracting officers 30 days to notify contractors, which must then certify full compliance by the 180-day deadline.