While initially announced in May 2023, the first F-16 fighter jets arrived in Ukraine Thursday. F-16's can be equipped to carry nuclear weapons, although the ones sent to Ukraine are not reportedly so-equipped. Putin stated in May that Ukrainian F-16's will be seen as a nuclear threat.

A letter from President Joe Biden to the Speaker of the House Mike Johnson and President of the Senate Kamala Harris, issued Monday, detailed an amendment to a 1958 agreement between the U.S. and the U.K. regarding nuclear arms cooperation.

Monday's amendment to the 1958 agreement removed the expiration date for certain provisions of the original agreement so as to permit the transfer of certain nuclear-related equipment and material between the two countries, conceivably for potential use against Russia.

After detailing what the 1958 agreement permitted, the letter detailed what is now being expanded upon with the new amendment.

"The Amendment's revisions to Article II(B) of the 1958 Agreement clarify that these same types of information may be exchanged related to atomic weapons, including special nuclear materials properties and production or processing technology, as is determined necessary to improve the recipient's atomic weapon design, development, or fabrication capability," the White House letter said.

Biden's letter said that the U.S. will continue to assist the U.K. in maintaining a credible nuclear deterrent.