Russia was supposed to guarantee the disposal of Syria's stockpile of chemical weapons in September 2013, but President Bashar al-Assad's regime is suspected of conducting repeated gas attacks since then.

"The first thing we have to look at is why are chemical weapons still being used at all when Russia was the framework guarantor of removing all the chemical weapons," Mattis said at the Pentagon in a meeting with his Qatari counterpart.

"Working with our allies and our partners from NATO to Qatar and elsewhere, we are going to address this issue ... I don't rule out anything right now."

Syria has been accused multiple times of using toxic weapons including sarin gas in the country's seven-year war, which has killed more than 350,000 people.

Backed by Moscow, Assad has waged a seven-week assault on Ghouta that has killed more than 1,700 civilians and left Islamist rebels cornered in their last holdout of Douma, Ghouta's largest town.

Dozens of people died April 4, 2017 after regime warplanes struck the rebel-held town of Khan Sheikhun in Idlib, with medical sources reporting patients suffering from symptoms consistent with a chemical attack.

In retaliation for that attack, US President Donald Trump unleashed strikes by Tomahawk missiles against the regime's Shayrat airbase overnight April 6-7.