Qatar's Emir Sheikh Tamim Bin Hamad Al-Thani welcomes U.S. Defense Secretary James Mattis (C) at his residence, the Sea Palace, in Doha, Qatar April 22, 2017. REUTERS/Jonathan Ernst

Qatar's Minister of Defence Khalid bin Mohammad Al-Attiyah (R) welcomes U.S. Defence Secretary James Mattis (C) and U.S. Ambassador to Qatar Dana Shell Smith (L) at his residence in Doha, Qatar April 22, 2017.

U.S. Defense Secretary James Mattis (R) greets an airman as he boards a U.S. Air Force C-17 for a day trip to a U.S. military base in Djibouti from Doha, Qatar April 23, 2017.

U.S. Defense Secretary James Mattis (C) is greeted by U.S. Ambassador to Qatar Dana Shell Smith (L) and Qatar Ministry of Defense Director of International Cooperation General Abdullah Jumaan al Hamad (R) as he arrives at Al Udeid Air Base in Doha, Qatar April 21, 2017.

Qatar's Minister of Defence Khalid bin Mohammad Al-Attiyah (L) welcomes U.S. Defence Secretary James Mattis (R) at his residence in Doha, Qatar April 22, 2017.

A Qatari soldier stands guard as Qatar's Minister of Defense Khalid bin Mohammad Al-Attiyah welcomes U.S. Defence Secretary James Mattis with a poolside dinner at his residence in Doha, Qatar April 22, 2017.

Qatar's Emir Sheikh Tamim Bin Hamad Al-Thani (R) welcomes U.S. Defense Secretary James Mattis (2nd L) and U.S. Ambassador to Qatar Dana Shell Smith (L) at his residence, the Sea Palace, in Doha, Qatar April 22, 2017.

Qatar's Minister of Defence Khalid bin Mohammad Al-Attiyah (R) welcomes U.S. Defence Secretary James Mattis (2nd L) and U.S. Ambassador to Qatar Dana Shell Smith (L) at his residence in Doha, Qatar April 22, 2017.

U.S. Defense Secretary James Mattis (C) is greeted by military dignitaries as he arrives at Al Udeid Air Base in Doha, Qatar April 21, 2017.

Qatar's Minister of Defense Khalid bin Mohammad Al-Attiyah (center right, in robe), U.S. Defence Secretary James Mattis (centre left, in dark suit) and U.S. Ambassador to Qatar Dana Shell Smith (R) stand for the playing of their national anthems before a dinner at the minister's residence in Doha, Qatar April 22, 2017.

U.S. Defense Secretary James Mattis boards a U.S. Air Force C-17 for a day trip to a U.S. military base in Djibouti from Doha, Qatar April 23, 2017.

Palace staff member waits to escort U.S. Defense Secretary James Mattis to meet with Qatar's Emir Sheikh Tamim Bin Hamad Al-Thani at his residence, the Sea Palace, in Doha, Qatar April 22, 2017.

Qatar's Emir Sheikh Tamim Bin Hamad Al-Thani (R) welcomes U.S. Defense Secretary James Mattis (L) at his residence, the Sea Palace, in Doha, Qatar April 22, 2017.