John Bacon, and Jim Michaels

USA TODAY

A Kurdish-led Syrian coalition backed by the U.S. announced Sunday the start of a campaign to liberate Raqqa, the Islamic State's de facto capital, amid hopes the battle will end the militant group's self-proclaimed caliphate in the Middle East.

The announcement by the Syria Democratic Forces comes three weeks after U.S.-backed Iraqi forces launched a campaign to retake the Iraqi city of Mosul from the Islamic State, also known as ISIL or ISIS. Both cities are crucial in the strategy to weaken the extremist group's grip in the region.

The Pentagon had pushed for the Raqqa offensive to start soon — while the Mosul operation is ongoing — to pressure the Islamic State in both cities at the same time. Simultaneous operations would force the Islamic State to defend both strongholds and capitalize on momentum against the militants in Iraq.

Defense Secretary Ash Carter welcomed word that the operation to free Raqqa "from ISIL's barbaric grip" was underway.

"The effort to isolate, and ultimately liberate, Raqqa marks the next step in our coalition campaign plan," Carter said Sunday in a statement. "As in Mosul, the fight will not be easy and there is hard work ahead, but it is necessary to ... disrupt the group's ability to carry out terror attacks against the United States, our allies and our partners."

Still, any assault to push the militants out of Raqqa is weeks or months away. The Pentagon said it is still training some of the forces that will take part in the operation, and the forces were still attempting to "isolate" the city, which is necessary before forces enter Raqqa and begin clearing operations.

In the Mosul campaign, the U.S.-led coalition is supporting Iraq's armed forces, which are relatively well organized and equipped with tanks and other equipment. By contrast the Syrian Democratic Forces are a loose confederation of tribal and other groups. They are lightly armed.

The Pentagon has decided to support the Raqqa operation, despite concerns from Turkey. The bulk of the 30,000 Syrian opposition force are Kurds, a fact that has made Turkey wary. Turkey has been battling Kurdish separatists within its own borders and has been concerned about their growing strength in neighboring Syria.

On Sunday, Marine Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, met with his counterpart in Turkey, Gen. Hulusi Akar. "The two leaders also agreed to continue to consult closely on the coalition plan to seize and hold Raqqa," a statement from Dunford's office said.

The Pentagon has said it is seeking to broaden the force to include more Arab fighters.

In Mosul, Iraqi military forces and their partners on Sunday continued the slow, methodical process of liberating neighborhoods east of the city block by block. The force of around 100,000 soldiers, security forces, Kurds and Shiite militias are backed by a U.S.-led coalition providing airstrikes, training and other support.

The densely populated neighborhoods, however, render airstrikes a threat to civilians and thus impractical.

"That's why we are carrying out the toughest urban warfare that any force in the world could undertake," Iraqi Counter Terrorism Service spokesman Sabah al-Numani told Reuters. Islamic State fighters continue to attack elsewhere in the country, with bombings that killed at least 20 people, the Associated Press reported.

U.S.-led coalition plans to pressure ISIL in both Mosul, Raqqa

Last week, the commander of Combined Joint Task Force-Operation Inherent Resolve stressed the need to begin the offensive in Raqqa. Lt. Gen. Stephen Townsend said U.S. intelligence has detected signs that Islamic State attacks against Western targets are being plotted from the city.

“Our intelligence feeds tell us that there is significant external operations attack planning going on,” he said.

Contributing: Steph Solis