Washington has sent conflicting signals to its longtime ally about its position on the diplomatic crisis which has seen Saudi Arabia and its allies impose sweeping sanctions on the emirate.
Last week, US President Donald Trump expressed support for the Saudi-led allegations against Qatar, charging that it had "historically been a funder of terrorism at a very high level".
But Pentagon and State Department officials have since scrambled to reassure the emirate, which houses the largest US airbase in the Middle East and the command headquarters for US military operations in Iraq, Syria and Afghanistan.
"The USD 12-billion sale will give Qatar a state-of-the- art capability and increase security cooperation and interoperability between the United States and Qatar," the Pentagon said.
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It did not provide additional details on the sale but Bloomberg reported it could involve as many as 36 warplanes.
Attiyah told the official Qatar News Agency the agreement would boost the emirate's defences and create 60,000 jobs in the United States.
"We believe that this agreement will propel Qatar's ability to provide for its own security, while also reducing the burden placed upon the United States military in conducting operations against violent extremism," he said.
The vessels docked in Hamad Port, south of the capital Doha, yesterday, the Qatari defence ministry said.
Mattis and Attiyah also discussed the US-led military campaign against the Islamic State jihadist group and "the importance of de-escalating tensions so all partners in the Gulf region can focus on next steps in meeting common goals".
Washington has voiced growing concern about the impact of the diplomatic crisis on its military operations against IS, just as they come to a climax in the jihadists' Iraq and Syria bastions, Mosul and Raqa.
At a Washington news conference with Tillerson yesterday, Saudi Foreign Minister Adel al-Jubeir denied the sanctions amounted to a blockade, insisting it was the kingdom's sovereign right to close its airspace to Qatari aircraft and seal the emirate's sole land border.
Saudi Arabia and its allies -- led by the United Arab Emirates, Egypt and Bahrain -- have said there can be no mending of ties until Qatar ends its support for groups like the Muslim Brotherhood, which the four governments blacklist as a "terrorist" group.
Turkey has launched a diplomatic push to try to resolve the crisis on behalf of its ally, throwing its support behind a UN-backed mediation effort by Kuwait, one of the Gulf states which did not join the Saudi-led sanctions.
Foreign Minister Mevlut Cavusoglu was in Kuwait today after talks in Qatar yesterday. He is due to fly on to Saudi Arabia tomorrow.
Turkey has been caught in a delicate balancing act. While standing squarely behind its ally, it has been anxious not to antagonise key regional power Saudi Arabia.
The United Nations has expressed mounting concern about the humanitarian impact of the crisis.
As well as severing economic and political ties, the four governments ordered Qataris out within 14 days and called home their own citizens.
Bahrain and the UAE have also banned expressions of sympathy for Qatar.
Bahrain announced yesterday that it had detained a citizen for sympathising with Qatar on social media.
"I am alarmed about the possible impact on many people's human rights," UN human rights chief Zeid Ra'ad Al Hussein said.