The U.S. has told the U.N. secretary-general that airstrikes against Islamic State group strongholds in Syria have begun because Syria's leaders have failed to stand up to the threat.

"The Syrian regime has shown that it cannot and will not confront these safe havens effectively itself," says Tuesday's letter from U.S. Ambassador Samantha Power to Ban Ki-moon.

"Accordingly, the United States has initiated necessary and proportionate military actions in Syria in order to eliminate the ongoing ISIL threat to Iraq," the letter says hours after the airstrikes began with the participation of five Arab nations.

The United States has warned of a lengthy campaign to defeat the extremists who have seized territory spanning the Syria-Iraq border.

Damascus has said the U.S. informed it beforehand that the strikes were coming.