"What is called moderate opposition sold to ISIL the innocent, beheaded U.S. journalist," Lahham writes to House Speaker John Boehner and House Minority Leader Nancy Pelosi, using an alternative acronym for ISIS. "There is nothing to prevent those groups from selling the U.S. weapons to ISIL as it is their proven common practice."
Lahham doesn't name the journalist, but it's the same accusation made by a spokesman for the family of Steven Sotloff, whose execution video was posted online by ISIS earlier this month.
In his letters to the U.S. Congress, Speaker Lahham argues that there is no such thing as moderate rebels and arming and training those selected groups "is a nonreversible action that will trigger a global negative chain reaction."
The White House is lobbying Congress to authorize $500 million to arm and train "vetted" moderate Syrian rebel groups to fight ISIS. The CIA has already been covertly training certain rebel groups fighting the Assad regime in countries bordering Syria, but this would be an expanded program. Saudi Arabia has already agreed to allow rebels to train in the country, according to U.S. officials.
Boehner said today that he hasn't seen Lahham's letter, but said, "I frankly, think the president’s request is a sound one. I think there’s a lot more that we need to be doing, but there’s no reason for us not to do what the president asked us to do." Pelosi's office said she hasn't received the letter, so can't comment on it.
The Obama administration has long resisted arming the moderates on a large scale with small and heavy weapons out of fear that arms given to the relatively weak and very fractured forces would fall into the hands of extremists. Moderate Syrian forces say the West's reluctance to arm them over the past three years of Syria's civil war has given rise to the radical Islamist elements in Syria whose wealthy patrons from the Gulf have funded more sophisticated weaponry.
The U.S. has carried out over 160 airstrikes against ISIS targets in Iraq and has warned that it will begin to target the group inside Syria where it has its base in the northern city of Raqqa. The Obama administration has said it is not coordinating with either the regime of President Bashar Assad, nor Iran, Syria's main backer.
Syria has warned that an American assault against ISIS inside Syria without coordination would violate their sovereignty.
ABC News' John Parkinson contributed to this report