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The United States and five Arab partner nations launched airstrikes against ISIS targets in Syria for the first time Monday, the Pentagon said. In a major escalation of the U.S. war against ISIS, the military planned to strike up to 20 targets, including fuel and weapons depots, training sites, troop encampments, command and control centers and the Sunni fighters' headquarters. The bombing began at approximately 8:30 p.m. ET Monday, a senior U.S. defense official told NBC News.

The London-based Syrian Observatory for Human Rights reported that airstrikes had been reported in ISIS' stronghold of Raqqa as well as in Tal Abyad, which is located near the Turkish border. NBC News could not immediately verify the group's account. ISIS is believed to have evacuated many of its sites ahead of the strikes.

Early Tuesday, the Pentagon confirmed that its partners in the airstrikes against ISIS included Jordan, Bahrain, Saudi Arabia, Qatar and the United Arab Emirates.

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The strikes are part of a military campaign the Obama administration authorized nearly two weeks ago to "degrade, and ultimately destroy" the ISIS fighters, who have killed thousands of people, beheaded Westerners — including two American journalists — and seized a broad swath of territory along the border of Syria and Iraq.

U.S. officials informed Syria's envoy to the United Nations on Monday of the possibility of strikes against ISIS targets inside the country, Syrian state TV reported.

The United States has previously stressed it would not coordinate with the government of President Bashar Assad in any way in its fight against ISIS. President Barack Obama's position has long been that he would like to see Assad leave power, particularly after using chemical weapons against his own people last year.

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US military & partner nation forces have begun striking ISIL targets in Syria using mix of fighters, bombers and Tomahawk missiles. — Rear Adm. John Kirby (@PentagonPresSec) September 23, 2014

- Jim Miklaszewski, Courtney Kube, Ammar Cheikh Omar and Daniel Arkin

Reuters contributed to this report.