It was also announced that Secretary of State Rex Tillerson will travel to Kuwait -- one of the countries angry at Qatar in the spat -- on July 10 at the invitation of Kuwait to discuss the row.
Mattis, who spoke with his Qatari counterpart Khaled bin Mohammed al-Attiyah by phone, discussed the status of operations against the Daesh (also known as ISIL or ISIS) group, AFP reported.
Qatar hosts a vital US-led command center at the Al-Udeid air base, where the anti-ISIL coalition launches raids.
"Secretary Mattis and Minister al-Attiyah affirmed their commitment to continued US-Qatar cooperation and deepening their strategic partnership," a Pentagon readout of the conversation stated.
Saudi Arabia is leading a four-country blockade of Qatar in the region's biggest crisis in years.
Mattis stressed the importance of de-escalating tensions "so all partners in the Persian Gulf region can focus on next steps in meeting common goals," the readout stated.