The United States says it has evidence chemical weapons have been used by the Syrian government. U.S. intelligence thinks it has happened "on a small scale" in the civil war there.

The President has previously said the use of chemical agents could shift his policy on intervention.

The White House says the UN needs to fully investigate. Washington has said any use of chemical weapons would cross a "red line" in the ongoing conflict.

The White House has not specified what action Obama might take if he determines with certainty that Syria has used any chemical weapons.

The White House said the assessment that the Syrian regime had used chemical weapons, and specifically the chemical sarin, was based in part on physiological samples.

"The intelligence community has been assessing information for some time on this issue and the decision to reach this conclusion was made in the past 24 hours" US Defence Secretary Chuck Hagel told reporters in Abu Dhabi.

Hagel also said while the United States could not confirm the origin of the chemical weapons it believes any use would very likely have originated with the regime of President Bashar al-Assad.

It comes just days after the top military intelligence analyst in Israel claimed there was evidence that Syrian government forces have used chemical weapons in their fight against opposition rebels.

Brigadier General Itai Brun made the comments at a security conference in Tel Aviv saying he was 100% certain that nerve gas was used in the Aleppo area.

The comments come less than a week since France and Britain made similar assertions to the United Nations.