WASHINGTON Twenty or more Americans are missing following Palestinian Islamist group Hamas' attack on Israel over the weekend that left hundreds dead, U.S. national security adviser Jake Sullivan said on Tuesday.

Sullivan said it was not known how many of those still missing are being held as hostages.

"We believe that there are 20 or more Americans who at this point are missing, but I want to underscore and stress that does not mean necessarily that there are 20 or more American hostages. Just that is the number who are currently unaccounted for," Sullivan told reporters.

(Reporting by Steve Holland; writing by Kanishka Singh; Editing by Leslie Adler)