HUKOUMIYA, Syria/BEIRUT (Reuters) - The U.S.-backed Syrian Democratic Forces (SDF) said on Tuesday it had begun a battle to capture Raqqa, Islamic State's de facto capital in Syria, launching attacks from the east, west and north of the city.

In a phone interview with Reuters from Syria, SDF spokesman Talal Silo said the operation started on Monday and the fighting would be "fierce because Daesh (Islamic State) will die to defend their so-called capital".

Islamic State captured the city from rebel groups in 2014 and has used it as an operations base to plan attacks in the West. The assault on Raqqa will pile more pressure on Islamic State's self-declared "caliphate" with the group facing defeat in the Iraqi city of Mosul and being forced into retreat across much of Syria.

"The coalition has a big role in the success of the operations. In addition to warplanes, there are coalition forces working side by side with the Syrian Democratic Forces," Silo said.

The Syrian Observatory for Human Rights, a Britain-based organization that reports on the war, earlier said the SDF attacked the eastern edge of Raqqa and a military base on the northern outskirts of the city on Tuesday.

The Kurdish YPG, part of the SDF, told Reuters on Saturday the assault on Raqqa was expected to start in a few days.

"It started today at dawn," Observatory Director Rami Abdulrahman said. "They have reached the city but they have not entered any of its buildings."

The attack on the al-Mashlab district and on the Division 17 base around 1 km to the north of the city center followed heavy overnight air strikes, the Observatory said.

The SDF has been working to encircle Raqqa since November in an offensive backed by the U.S.-led coalition that is also fighting Islamic State in Iraq, where the jihadist group is on the brink of defeat in the city of Mosul.

The U.S.-led coalition has said 3,000 to 4,000 Islamic State fighters are thought to be holed up in Raqqa city, where they have erected defenses against the anticipated assault. The city is about 90 km (56 miles) from the border with Turkey.

The United States said on Tuesday it had started distributing arms to the YPG to help take Raqqa, part of a plan that has angered NATO-ally Turkey, which is worried by growing Kurdish influence in northern Syria.

(Reporting by Rodi Said in Syria, Tom Perry and Ellen Francis in Beirut; Writing by Tom Perry; Editing by Janet Lawrence)