Saudi Arabia's Deputy Crown Prince and Defense Minister Mohammed bin Salman (2nd L) takes his seat to meet with U.S. Defense Secretary James Mattis and his delegation in Riyadh, Saudi Arabia April 19, 2017.

U.S. Defense Secretary James Mattis departs after meeting with Saudi Arabia's Deputy Crown Prince and Defense Minister Mohammed bin Salman at the Ministry of Defense in Riyadh, Saudi Arabia April 19, 2017.

U.S. Defense Secretary James Mattis greets Turkey's Defense Minister Fikri Isik at the Pentagon in Arlington, VA, U.S. April 13, 2017.

U.S. Defense Secretary James Mattis greets Egyptian President Abdel Fattah al-Sisi at the Pentagon in Arlington, VA, U.S. April 5, 2017.

U.S. Defense Secretary James Mattis chats with reporters after a media briefing at the Pentagon in Washington, U.S., April 11, 2017.

U.S. Defense Secretary James Mattis gestures to the media at the Pentagon in Washington, U.S., April 11, 2017.

U.S. Defense Secretary James Mattis stands with Turkey's Defense Minister Fikri Isik at the Pentagon in Arlington, VA, U.S. April 13, 2017.

U.S. Defense Secretary James Mattis gestures to the media at the Pentagon in Washington, U.S., April 11, 2017.

U.S. Defense Secretary James Mattis briefs reporters aboard his plane en route to the Middle East after departing from Joint Base Andrews, Maryland, U.S. April 17, 2017. Picture taken April 17, 2017.

U.S. Defense Secretary James Mattis is seen at the Pentagon in Arlington, VA, U.S. April 13, 2017. REUTERS/Eric Thayer

U.S. Defense Secretary James Mattis (L) and Army Gen. Joseph Votel, commander of U.S. Central Command, leave after a media briefing at the Pentagon in Washington, U.S., April 11, 2017.

U.S. Defense Secretary James Mattis departs after meeting with Saudi Arabia's Deputy Crown Prince and Defense Minister Mohammed bin Salman at the Ministry of Defense in Riyadh, Saudi Arabia April 19, 2017.

U.S. Defense Secretary James Mattis arrives at the start of a week-long Middle East visit at King Salman Air Base, Riyadh, Saudi Arabia April 18, 2017.

U.S. Defense Secretary James Mattis (R) is greeted by Saudi Armed Forces Chief of Joint Staff General Abdul Rahman Al Banyan (L) upon his arrival at King Salman Air Base, Riyadh, Saudi Arabia April 18, 2017.

Saudi Arabia's King Salman bin Abdulaziz Al-Saud, flanked by an interpreter, welcomes U.S. Defense Secretary James Mattis in Riyadh, Saudi Arabia April 19, 2017.

Saudi Arabia's Deputy Crown Prince and Defense Minister Mohammed bin Salman (3rd L, far side of table) meets with U.S. Defense Secretary James Mattis (3rd L, near side of table) and his delegation in Riyadh, Saudi Arabia April 19, 2017.

Saudi Arabia's Deputy Crown Prince and Defense Minister Mohammed bin Salman welcomes U.S. Defense Secretary James Mattis in Riyadh, Saudi Arabia April 19, 2017.

U.S. Defense Secretary James Mattis greets Turkey's Defense Minister Fikri Isik at the Pentagon in Arlington, VA, U.S. April 13, 2017.

U.S. Defense Secretary James Mattis greets Egyptian President Abdel Fattah al-Sisi at the Pentagon in Arlington, VA, U.S. April 5, 2017.

U.S. Defense Secretary James Mattis chats with reporters after a media briefing at the Pentagon in Washington, U.S., April 11, 2017.

U.S. Defense Secretary James Mattis briefs reporters aboard his plane en route to the Middle East after departing from Joint Base Andrews, Maryland, U.S. April 17, 2017. Picture taken April 17, 2017.

U.S. Defense Secretary James Mattis (L) and Army Gen. Joseph Votel, commander of U.S. Central Command, brief the media at the Pentagon in Washington, U.S., April 11, 2017.