Mattis secretly advocating Trump arms sale amid growing opposition
The sale would include a sale of up to 50 F-35 fighter jets, almost 20 Reaper drones, and about 14,000 bombs and munitions.
The report, which cited three people familiar with Mattis' involvement in the case, comes amid growing opposition from Congress over concerns about the UAE's human rights record.
The UAE is also heavily involved in both Yemen and Libya's conflicts, with rights groups warning earlier this month that the arms sale would "fuel continued civilian harm and further exacerbate these humanitarian crises".
US Senators are also concerned that the sale could erode the military edge of Israel in the Middle East.
Mattis reportedly became involved in the effort to swing lawmakers earlier this week, ahead of a vote on bipartisan legislation to block the sale.
A source said that Senators with reservations about the deal were advised by the UAE's US ambassador, Yousef Al-Otaiba, to speak to Mattis.
Mattis and Trump publicly fell out in 2018 when the former defence chief resigned over disagreements with the president, in part due to troop movements in the Middle East.
In June, Mattis also slammed Trump for his divisive brand of politics, calling him a threat to the constitution.
Mattis, however, appears to be willing to put his rift with the president aside to advocate on behalf of the UAE.
Mattis has previously referred to the UAE affectionately as "Little Sparta", and has advised the Gulf state's military in an unpaid capacity.
Mattis also has ties to the arms industry through his role as a senior counselor at The Cohen Group, which could benefit from the deal's approval.
Congress is due to vote on the deal on Wednesday.
Republican Senate leader Mitch McConnell has said the protest bill has "no chance of obtaining a veto-proof majority".
However, the White House's eleventh hour lobbying of lawmakers suggests a level of worry about which way the vote will go.
"It's a little baffling to suggest that, now of all times, a protest gesture with no chance of obtaining a veto-proof majority is a valuable use of the Senate's time," McConnell said.
"The strategic realities dictate that Congress should not stand in the way of this sale," he added.