The United States has reportedly bolstered an armed group in the southern parts of Syria ahead of a surprise offensive that ousted President Bashar al-Assad.

In the first indication that Washington had prior knowledge of the offensive, the group known as Revolutionary Commando Army (RCA) revealed it had been told to scale-up its forces and “be ready” for an attack that could lead to the end of the Assad government, the Telegraph reported.

The RCA fighters, trained by Britain and the US, were told "this is your moment" during a briefing by US Special Forces stationed in the Arab country before Assad was toppled on December 8, the report noted.

The RCA fighters said Washington had prior knowledge of the offensive, which was mainly led by Hayat Tahrir al-Sham (HTS). The RCA was told to increase its forces and prepare for a major attack that could "end" the Syrian government.

Capt. Bashar al-Mashadani, an RCA commander said in the weeks preceding the offensive, the RCA's ranks were expanded by smaller "freelance" units, all of which were briefed at the US al-Tanf air base.

“They did not tell us how it would happen,” al-Mashadani told The Telegraph from a former Syrian army air base on the outskirts of the city of Palmyra.

“We were just told: ‘Everything is about to change. This is your moment. Either Assad will fall, or you will fall.’ But they did not say when or where, they just told us to be ready.”

The RCA is an armed group established by defected Syrian Arab Army (SAA) troops and is headquartered in the al-Tanf area, near the Syria-Jordan-Iraq border area, in southern Syria.

US forces are also stationed in the al-Tanf area, where they claim to be fighting the Daesh Takfiri terrorist group in the region.