The United Arab Emirates denied on Tuesday that it had supplied a Libyan armed group with powerful American-made missiles, after a Democratic senator warned that Washington could halt all arms sales to the United Arab Emirates.

On Friday the State Department said it had opened an investigation into how the weapons — four American-made Javelin armor-piercing missiles, normally sold only to close American allies — had found their way into a cache of arms discovered last week at a rebel base south of Tripoli.

Markings on the missile crates indicated they were part of a consignment sold to the United Arab Emirates in 2008, The New York Times reported. If confirmed, the transfer of those weapons to Libya would constitute “a serious violation of United States law,” Senator Robert Menendez, the ranking Democrat on the Foreign Relations Committee, said in a letter to Secretary of State Mike Pompeo on Tuesday.

“You are surely aware that if these allegations prove true you may be obligated by law to terminate all arms sales to the United Arab Emirates,” Mr. Menendez wrote.