The countries have long worried about being sucked into regional war as they juggle relations with Washington, their security guarantor, while seeking to improve ties with Tehran.

The Pentagon already had begun taking measures to strengthen its firepower in the Middle East and protect U.S. bases there.

President Trump said Monday the Iranian government offered "early notice" about a series of missile strikes that targeted the U.S. Al Udeid Air Base in Qatar, downplaying Iran's response to the U.S. bombing of three Iranian nuclear sites over the weekend as "very weak."