Republican and Democratic representatives in the US House of Representatives urged President Barack Obama to stop trying to overthrow Syrian leader Bashar al-Assad and focus all efforts on fighting the Islamic State (ISIL) terrorist group.

On Friday, Democrat Tulsi Gabbard and Republican Austin Scott introduced legislation to end an "illegal war" to overthrow Assad.

"Working to remove Assad at this stage is counter-productive to what I believe our primary mission should be," Austin was quoted as saying by AP.

"The US is waging two wars in Syria. The first is the war against ISIS and other Islamic extremists, which Congress authorized after the terrorist attack on 9/11. The second war is the illegal war to overthrow the Syrian government of Assad," Gabbard said.

Since 2013, the CIA has trained about 10,000 fighters within a covert program to train, arm and fund a moderate opposition to Assad. The secret program is the only step the US is taking against government forces in Syria.

She underscored that Congress has not authorized the CIA’s efforts to support Syrian opposition groups.

The effort to overthrow Assad is counterproductive because it is helping ISIL advance, Gabbard added. If ISIL seizes the Syrian Army’s weapons and infrastructure it would become even more dangerous.

"I don't think Assad should be removed," Gabbard told CNN. "If Assad is removed and overthrown, ISIL, al-Qaeda, al-Nusra, these Islamic extremist groups will walk straight in and take over all of Syria… they will be even stronger."

In Syria, the US may repeat its mistakes of the past, like the toppling of Saddam Hussein in Iraq or Muammar Gaddafi in Libya.

"The results of those two failed efforts of regime change and the following nation-building have been absolute, not only have they been failures, but they've actually worked to strengthen our enemy," she added.