The United States says forces loyal to the the Syrian President Bashar al-Assad have used chemical weapons against rebel fighters in Syria, and is to supply direct military assistance to the opposition, the White House says.
The new intelligence assessment followed President Barack Obama's demand for conclusive proof that chemical weapons had been deployed.
Deputy National Security Advisor Ben Rhodes said the intelligence community estimated 100 to 150 people had died from detected chemical weapons attacks in Syria to date, though he said casualty data is likely to be incomplete.
He said Mr Obama had decided to provide "direct military support" to the opposition.
Mr Rhodes said this would be different in "both scope and scale" to what had been authorized before, which included non-lethal equipment such as night-vision goggles and body armour.
However Mr Rhodes would not specify whether the new help would include weapons, which would mark a reversal of Me Obama's resistance to arming the rebels.
Mr Obama has previously made it clear he will not put American troops in Syria.
The arrival of thousands of seasoned Iran-backed Hezbollah Shi'ite fighters to help President Assad combat the mainly Sunni rebellion has shifted momentum in the civil war, Reuters reports.
The United Nations said on Thursday at least 93,000 people had been killed in the war.
The White House announcement followed intensive deliberations between Mr Obama and his national security aides on Syria amid mounting pressure at home and abroad for him to act more forcefully in the conflict, including a sharp critique of his policy from former President Bill Clinton.