President Barack Obama admitted that he has seen evidence of chemical weapons being used in Syria.

However, he said that after speaking with the prime minister of Turkey that it was important to get more specific details about the attacks. His statement comes as residents of a town in northern Syria told the BBC that Syrian troops dropped poison gas canisters from helicopters.

“Our militaries are constantly sharing information. We have seen evidence of the use of chemical weapons inside Syria,” President Obama said in a press conference. “Those chemical weapons inside of Syria also threaten our security over the long term as well as [that of] our allies and friends and neighbors.”

Allegations of chemical weapon use have been leveled at the Syrian government since March. Syrian officials continue to insist that they have not used them despite the mounting evidence.

The United Nations has assembled a 15 member team to investigate the allegations of chemical weapons use but the Syrian government refuses to let the team into the country.