There is "limited but growing" evidence that Syrian government troops have used chemical weapons, Prime Minister David Cameron says.
"It is extremely serious, this is a war crime," the PM told the BBC.
For over two years, a violent conflict has raged in Syria between government forces and rebels who want the country's leaders out.
If it's confirmed that chemical weapons have been used, it may mean other countries decide to get involved.
Chemical weapons, such as poison gases, are banned under international law because of the harm they can cause to people over a wide area.
Experts aren't sure yet
On Thursday, America's government said they believed "with varying degrees of confidence" that Syria was using a chemical weapon. This means that not all the experts agree that there is enough evidence yet.
US experts said that a poison gas called sarin had been used on a "small scale", but did not give details of where or when it had been used.
President Obama said that if this was the case, it would mean that a "red line" had been crossed and that America might have to intervene in Syria.
Syria's government and rebels have accused each other of using chemical weapons. A team from the United Nations is trying to enter Syria to investigate.