Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE is pressing lawmakers to reject a Senate proposal that looks to end U.S. support for Saudi Arabia in its war in Yemen, The Wall Street Journal reported.

Mattis, in a Wednesday letter to Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellOcasio-Cortez to voters: Tell McConnell 'he is playing with fire' with Ginsburg's seat McConnell locks down key GOP votes in Supreme Court fight Video shows NYC subway station renamed after Ruth Bader Ginsburg MORE (R-Ky.), said the bipartisan proposal would potentially harm Washington’s partnership with Saudi Arabia and increase the chances of a war with Iran, according to the Journal.

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The United States has provided support for the Saudi campaign in Yemen’s three-year civil war, started after Iranian-backed Houthi rebels took over the country’s capital. Saudi Arabia, in response, formed a coalition to intervene on behalf of Yemen’s President Abd-Rabbu Mansour Hadi, who fled to the south.

The United States has given the Saudis weapons and helped with limited intelligence and logistics, such as air refueling of jets.

“New restrictions on this limited U.S. military support could increase civilian casualties, jeopardize cooperation with our partners on counterterrorism, and reduce our influence with the Saudis — all of which would further exacerbate the situation and humanitarian crisis,” Mattis wrote in the letter, according to the Journal.

The letter was sent in response to a potential vote next week on whether or not to end U.S. military involvement in Yemen.

Sen. Bernie Sanders (I-Vt.) told reporters Wednesday that his resolution with Sens. Mike Lee (R-Utah) and Chris Murphy (D-Conn.) will be brought up before Congress leaves for a two-week recess.

The resolution would remove U.S. forces not involved in fighting al Qaeda or related groups from Yemen within 30 days.

The lawmakers introduced the legislation in late February, over concerns that the Pentagon’s involvement is unconstitutional because Congress did not have a say in entering the fight.

Mattis writes that withdrawing U.S. support “would embolden Iran to increase its support to the Houthis, enabling further ballistic missile strikes on Saudi Arabia and threatening vital shipping lanes in the Red Sea, thereby raising the risk of a regional conflict.”

The Pentagon has made no secret of its moves to stop the resolution, which could come up for a vote when Saudi Arabia’s Crown Prince Mohammed bin Salman is in Washington next week to meet with President Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE and top U.S. officials.

The Pentagon sees Saudi Arabia as an ally in the fight against al Qaeda and the Islamic State in Iraq and Syria. The administration also hopes Riyadh will help curtail Iran’s nuclear and military advancements.

Critics of U.S. military air support to Saudi Arabia, however, argue the civil war has wiped out thousands of Yemen's civilians, in part due to the Saudi coalition’s airstrikes. The United Nations has repeatedly warned of the rising civilian death toll, and Mattis himself was forced to defend U.S. efforts to prevent the casualties.