U.S. backed forces began an offensive to rout the Islamic State from Raqqa, their de facto capital in Syria, the American-led coalition announced Tuesday.

The offensive will be difficult but will deal a decisive blow to the terror group, which has been losing ground over the past year in both Iraq and Syria, said Lt. Gen. Steve Townsend, the coalition commander.

Raqqa is the remaining stronghold of the terror group’s so-called caliphate. Iraqi forces are close to clearing Mosul, Iraq’s second largest city, from Islamic State control.

"It's hard to convince new recruits that ISIS is a winning cause when they just lost their twin 'capitals' in both Iraq and Syria," Townsend said in a statement.

The offensive is being led by a coalition of Kurdish and Arab forces, which are backed by coalition advisers and airstrikes. The Trump administration recently announced it would begin supplying the Kurdish forces with arms and equipment to help in the offensive against the Islamic State, which is also called ISIS.

Turkey, which is helping in the anti-ISIS fight, is opposed to the support for the Kurds because of fears the ethnic group will push for its long-time goal of carving out an independent state in parts of Turkey, Iraq and Syria.

The U.S.-backed forces have been working to surround the city since November to cut off supply routes in and out of the city in northeastern Syria.

The Pentagon announced recently that the coalition’s tactics are designed to annihilate the terror group so that its fighters cannot escape to establish strongholds elsewhere or launch terror attacks against Western targets.

"We all saw the heinous attack in Manchester, England," Townsend said in the statement. "ISIS threatens all of our nations, not just Iraq and Syria, but in our own homelands as well. This cannot stand."