US President Barack Obama says reports of a chemical weapons attack in Syria represent a "big event of grave concern".
Hundreds of people are reported to have been killed in a suspected chemical weapons attack on Wednesday, just outside the Syrian capital Damascus.
There are calls for UN weapons inspectors who are already in the city, to be allowed to investigate the site.
But there's no sign yet that Syria's government will allow this to happen.
Last year President Obama said the use of chemical weapons in Syria would cross a "red line" and force a tough US response.
Also on Friday, the United Nations announced that the number of child refugees that have fled Syria had reached one million.
Situation is unclear
It's extremely difficult to work out what's happened in Syria as both sides blame each other for attacks.
Rebel fighters claim that the Syrian government carried out the attack, but the government denies this, saying it was the rebels themselves.
If it's confirmed that chemical weapons have been used, it may mean other countries decide to get involved in Syria's war.
Chemical weapons, such as poison gases, are banned under international law because of the harm they can cause to people over a wide area.