RAQQA, SYRIA / WASHINGTON - U.S.-backed Syrian forces stormed into Islamic State's de-facto capital of Raqqa on Tuesday, triggering intense combat and signaling the start of what the U.S.-led coalition says will ultimately be the end of IS control in the besieged city.

"We declare today the beginning of the great battle to liberate the city of Raqqa, the alleged capital of terrorism and terrorists," Talal Silo, a spokesman for the Kurdish-led Syrian Democratic Forces, told reporters at a press conference in northern Syria.

"Morale is high and military readiness to implement the military plan is complete, in coordination with the U.S.-led coalition," Silo said.

Captured in 2014

Raqqa was captured by IS in January 2014 and has since become the base for the group's hierarchy. The SDF has advanced for months steadily southward toward Raqqa, clearing out IS along the way with the help of U.S. bombing efforts.

Syrian Democratic Forces (SDF) spokesman Talal Sil Syrian Democratic Forces (SDF) spokesman Talal Silo speaks during a news conference in Hukoumiya village in Raqqa, Syria, June 6, 2017. Syrian Democratic Forces (SDF) spokesman Talal Silo speaks during a news conference in Hukoumiya village in Raqqa, Syria, June 6, 2017.

For its part, IS has vowed a last stand in Raqqa.

Tuesday's battle inside the city limits — a first in the monthslong battle — indicate the fight may be long and costly, SDF commanders say.

By nightfall, SDF commanders told VOA that forces had approached eastern neighborhoods of Raqqa while fierce skirmishes continued with IS fighters in the western and northern outskirts of the city.

"Our forces will continue pushing from these front lines in a process that is step-by-step," Jihan Sekh Ahmed, a spokeswoman for the SDF operation, told VOA. "This is based on a military plan that we have prepared after a long period of discussions."

There was no count of casualties, but Kurdish commanders said they have heard that IS is holding thousands of civilians as human shields. IS reportedly cut off electrical service and shuttered internet cafes in the city. Unconfirmed estimates put the city's current population at about 200,000.

"Those who try to escape once SDF forces advance further into the city risk being killed from mines and IS snipers as well as airstrikes," Thomas Garofalo, International Rescue Committee's Middle East director of public affairs, said in a statement.

The U.K.-based Syrian Observatory for Human Rights said the SDF moved into al-Mashlab neighborhood of Raqqa in the evening and took control of some buildings as IS fighters began to withdraw.

Former Syrian military base

In the northern outskirts of the city, SDF is targeting an IS-held former Syrian military base known as Division 17.

Kurdish commanders told VOA that SDF forces seized several farms and silos nearby and were advancing toward the military base with aerial support from the coalition.

The anti-IS monitoring group Raqqa is Being Slaughtered Silently, which has observers in the city, said airstrikes hit IS targets in the south and center of the city to prevent fighters from sending reinforcements to the eastern front line. The group on its Twitter account accused the coalition of targeting two bridges and killing 17 civilians.

Raqqa, Syria

SDF commanders said they are closely monitoring the fate of residents.

"We will open several safety corridors for the people in the coming days," SDF spokeswoman Ahmed told VOA. "We expect many civilians to come to our territories as fighting continues. We will take them to safety before we return stability to Raqqa."

Lt. Gen. Steve Townsend, who commands the U.S.-led coalition fighting Islamic State in Syria and Iraq, said the battle for Raqqa will be long and difficult, but the coalition will "deliver a decisive blow" in Raqqa, according to a coalition statement.

"It's hard to convince new recruits that IS is a winning cause when they just lost their twin capitals in both Iraq and Syria," Townsend said, referring to Raqqa and Mosul, Iraq, where battles against IS continue.

Coalition support

The coalition will continue providing equipment, training, intelligence and logistical support to the SDF during the Raqqa offensive, Townsend said.

SDF commanders said the level of coalition support and arms supply would depend on the durability of the forces and difficulty of the battle.

FILE - Syrian Democratic Forces (SDF) fighters ho FILE - Syrian Democratic Forces (SDF) fighters hold up their weapons in the north of Raqqa city, Syria. FILE - Syrian Democratic Forces (SDF) fighters hold up their weapons in the north of Raqqa city, Syria.

"Some of the coalition weapons arrived before the start of the operation and more are on the way," Nasser Haj Mansour, an adviser to the SDF, told VOA. "SDF and the coalition have been working together against IS for two years now and there is a high degree of confidence in each other at this vital point."

The fate of Raqqa post-IS has been the subject of intense speculation and tension among the various entities fighting in Syria.

Turkey opposes the role of the SDF and its large contingent of Kurdish fighters that Turkish officials consider terrorists. The Syrian regime and its allied Russian and Iranian forces have also set their sights on Raqqa but have yet to move on the city.

The U.S.-led coalition said that when Raqqa is retaken, the city will be turned over to local civilians and not held by the group.

Syria analyst David Lesch, a history professor and Mideast analyst at Trinity University in San Antonio, Texas, said the issue of who is in charge will be key going forward.

"Many, many questions still remain to be answered, and still will probably be the subject of a diplomatic battle in terms of who controls Raqqa once it is liberated," Lesch told VOA. "Will it be the Syrian Democratic Forces? Will Turkey have a role? Will the Syrian regime have a role? What will [regime ally] Russia do in this case?"