How can chemical weapons attacks in Syria be stopped?

A ''big price to pay''. That is the warning from the US president Donald Trump to Syrian leader Bashar al-Assad and his backers Iran and Russia.

It was made in response to a suspected chemical weapons attack in eastern Ghouta near Damascus on Saturday.

At least 85 people were killed.

UN investigators have documented at least 33 alleged chemical weapons attacks in Syria since 2011. They say 27 of them were carried out by government forces.

The worst was in Eastern Ghouta in August 2013 - a sarin gas attack that killed at least 1,000 people. But with Trump sending mixed signals on US troops in Syria, what more can he do to stop such attacks?