WASHINGTON (AFP) - President Barack Obama said on Wednesday he had not yet decided whether to attack Syria, but that any US action would be a warning that "it better not" use chemical arms again.

Mr Obama said that his government had concluded that President Bashar al-Assad's regime was behind the horrific attack on civilians in a Damascus suburb last week that killed hundreds of people.

But he clearly signaled that expected US punitive strikes would be limited in scope, and would not be an attempt to tip the balance between Assad's forces and rebels who have waged a bloody two-year civil war.

"I have not made a decision," Mr Obama told PBS NewsHour, when asked how close he was to signing off on US military action against Syria.

Mr Obama said however that he had concluded that "direct military engagement, involvement in the civil war in Syria, would not help the situation on the ground."

Given that limited vision for US military action in the country, Mr Obama was asked what would be the strategic rationale behind a US assault, likely to take place with cruise missiles fired from US and allied ships and submarines.

He said that by the end of the engagement, the Syrian government "will have received a pretty strong signal that it better not do it again."