U.S. sources told the Reuters news agency on Monday that congressional committees are holding up a plan to send U.S. weapons to rebels fighting Syrian President Bashar Al-Assad.

The delay is over fears that such deliveries will not be decisive and the arms might end up in the hands of Islamist rebel groups.

Both the Senate and House of Representatives intelligence committees have expressed reservations behind closed doors at the effort by President Barack Obama’s administration to support the insurgents by sending them military hardware, the sources told Reuters.

There has been growing pressure to arm the Syrian rebels, particularly since the U.S. government confirmed that the Syrian army used chemical weapons against rebel forces on multiple occasions, thus violating the “red line” set by Obama.

The U.S. said at the time that it will increase the “scope and scale” of its assistance to rebels in Syria in response to the use of chemical weapons by the Assad regime.

Rebels have already said they had received Russian-made “Konkurs” anti-tank missiles supplied by Saudi Arabia. Recent reports said that the Central Intelligence Agency has begun moving weapons to Jordan from a network of secret warehouses and plans to start arming small groups of vetted Syrian rebels within a month.

Democrats and Republicans on the committees, however, worry that weapons could reach factions like the Al-Nusra Front, which has pledged allegiance to Al-Qaeda chief Ayman al-Zawahiri.

Al-Nusra is one of 13 factions in the radical Islamist rebel council that announced its secession from the main opposition force and declared its own Islamic state in Aleppo. Members of the group have performed atrocities during the ongoing civil war, including publicly beheading a Catholic priest who was accused of collaborating with the Assad regime.

Funding that the administration advised the congressional committees it wanted to use to pay for arms deliveries to Assad’s opponents has been temporarily frozen, the sources told Reuters.

While the administration does not need specific congressional approval either through public legislation or some kind of legislative sanction process to move ahead with the weapons plan, administrations usually do not move ahead with programs like weapons deliveries to the Syrian opposition if one or both of the congressional intelligence committees express serious objections.