The United States has "no doubt" that the regime of Syrian President Bashar al-Assad was responsible for last week's chemical attack on a rebel-held town that left dozens dead, Pentagon chief Jim Mattis said Tuesday, reported AFP.

Mattis told reporters that Washington's military strategy in Syria had not changed even after its retaliatory missile strikes on a Syrian air base, noting "our priority remains the defeat" of the Islamic State group.

On the other hand a senior US administration official on Tuesday accused Russia of embarking on a misinformation campaign designed to "confuse the world" about what Washington says is the Syrian regime's use of sarin against its own people.

Speaking on condition of anonymity to discuss sensitive intelligence matters, the official said that Moscow was systematically trying to deflect blame away from the regime and onto rebels and the Islamic State group -- against all evidence.

The official added that US intelligence does not believe that the Islamic State group has sarin, the powerful chemical agent Washington is "confident" was used against the rebel-held town of Khan Sheikhun.

Meanwhile Britain, France and the United States have presented a new draft resolution to the UN Security Council demanding an investigation of the suspected chemical attack in Syria, the British ambassador said Tuesday.

The new measure would require "full cooperation with the investigation" of the apparent attack in the rebel-held town of Khan Sheikhun, Ambassador Matthew Rycroft said on Twitter.