Congress has quietly added a provision to the renewal of the National Defense Authorization Act that would leave the door open for the U.S. government to send anti-aircraft missiles to militants in Syria. This comes as President Obama is further lifting restrictions on providing weapons and other military support to Syrian rebels.

The Obama administration has previously been divided on the idea of providing anti-aircraft missiles to the Syrian opposition, which is dominated by extremist groups. The CIA has considered giving such weapons to rebels, yet other officials have expressed concerns that were the missiles to fall into the hands of al-Qaeda-aligned militants, they could subsequently be used to down civilian airliners.

This latest loosening of restrictions has been seen by some analysts as a way to further help Kurdish fighters combating ISIS in Raqqa. But the State Department has also stressed that it will continue supporting militants fighting the Syrian government, even after the recapture of Aleppo, meaning rebels could be recipients as well.

The 2017 NDAA bill will allow the U.S. to send man-portable air defense systems, or MANPADS, to vetted Syrian rebels, as long as several congressional committees are approved.

The new provision that permits this is buried in the conference report that accompanies the 2017 NDAA bill, which sits at a massive 3,076 pages in length. A congressional staffer who is familiar with the issue notified AlterNet of the quiet addition to the bill, which was made in a meeting of the bicameral conference committee.

Rep. Ted Yoho, a Republican from Florida, proposed an amendment to the 2017 NDAA bill in May that would prohibit the transfer of MANPADS to any entity in Syria. His amendment was adopted on a voice vote. The conference report acknowledged this House amendment. It added, however, that the Senate bill did not contain such a provision. AlterNet spoke with a separate congressional staffer who has worked on Syria policy.

The staffer, who asked to remain anonymous, said the provision in the final version of the NDAA was a compromise made in the congressional conference committee. While he and his colleagues wanted an outright ban, the staffer said the office saw the compromise as a small victory. He noted that previous versions of the NDAA did not mention MANPADS at all, and argued some restrictions are better than none.

The staffer could not say who in the conference committee opposed the outright ban on anti-aircraft missile transfers. John McCain, a hawkish Republican from Arizona, is the lead sponsor of the NDAA on the Senate side. In its report on the new MANPADS provision, the Washington Post pointed out that Sen. McCain has long called for more weapons for Syrian rebels.

In a statement released December 2, Rep. Yoho publicly took credit for introducing the compromise amendment into the final version of the NDAA, which he characterized as a success, "to help keep shoulder fired missiles out of the wrong hands in Syria."

Both of the congressional staffers interviewed by AlterNet, who work in separate offices for separate parties, warned that MANPADS are particularly dangerous because they are so portable. The missiles are very hard to track once they have been transferred abroad; they can fit in the trunk of a car or in a golf club bag and be taken to other countries.

The congressional staffers also said they have concerns about the vetting process for U.S.-backed Syrian rebels, which they warned is not as strong as they believe it should be. So-called moderate rebel groups in Syria frequently collaborate with al-Qaeda-aligned militants. Fighters from the Western-backed Free Syrian Army reportedly delivered kidnapped American journalist Theo Padnos to their ally Jabhat al-Nusra, Syria's al-Qaeda affiliate.

The Syrian opposition is dominated by these extremist groups, including Jabhat Fatah al-Sham (rebranded Syrian al-Qaeda), Ahrar al-Sham, Jaish al-Islam, and other hard-line Islamist militias. If such militants were to get their hands on MANPADS, they could be used to shoot down civilian aircraft. Moreover, if U.S.-backed Syrian rebels were to use anti-aircraft missiles to shoot down Russian planes, it could lead to a further escalation of tension between the two superpowers. Russia has more nuclear weapons than any other country.

While the U.S. government has, at least publicly, been divided on the issue, some Syrian rebels have had access to MANPADS for years, according to videos and photos posted to the internet by militants. How exactly the fighters got access to these weapons is unclear, but some analysts have speculated the anti-aircraft missiles came from Saudi Arabia, Qatar, or Turkey — Western allies that have provided heavy support to Syrian rebels.

More MANPADS could potentially be coming their way. Sec. 1224 of the NDAA conference report, the new provision in the 2017 renewal, stipulates that, if the government decides to use Pentagon funds to provide anti-aircraft missiles to vetted Syrian rebels, it may do so, under the condition that the secretaries of state and defense notify the congressional defense committees, the Senate Foreign Relations Committee, and the House Foreign Affairs Committee (or if 30 days elapses after those congressional committees are notified).

Lawmakers from both sides of the aisle have spoken out against further U.S. intervention in Syria. Rep. John Conyers, a Democrat from Michigan, has voiced concerns about sending anti-aircraft missiles to Syrian rebels. In May, Conyers joined Yoho and two dozen other congresspeople in penning a letter to President Obama warning that, "MANPADS would only lead to more violence, not only in Syria, but potentially around the world."

Similarly, Sen. Rand Paul, a libertarian-leaning Republican from Kentucky, has frequently expressed opposition to U.S. arming of Syrian rebels. "Most of the arms we’ve given to the so-called moderate rebels have wound up in the hands of ISIS, because ISIS simply takes it from them, or it’s given to them, or we mistakenly actually give it to some of the radicals," Paul said in 2014.

At its peak in 2015, the operation to arm and train Syrian rebels was the CIA's largest covert program, with a budget of nearly $1 billion per year. Documents released by NSA whistleblower Edward Snowden showed that $1 of every $15 in the CIA's overall budget went to operations in Syria. This was in addition to the billions of dollars in funding that came from the Western-backed authoritarian regimes in Saudi Arabia, Qatar and Turkey.