U.S. and European officials have determined that the Syrian government has deployed chemical weapons against rebels, the White House has confirmed.

The White House notified lawmakers on Thursday that they have determined chemical weapons have been used.

The White House previously acknowledged that chemical weapons had been likely deployed, but they needed further confirmation. Obama had called thepotential use of chemical weapons by Syrian President Bashar Assad a "red line" that would spur further action by the U.S.

The president "has said the use of chemical weapons would change his calculus and it has," said Deputy National Security Adviser Ben Rhodes.