ANKARA, Turkey (AP) — Turkey's military and the U.S.-backed coalition forces on Wednesday launched an operation to clear a Syrian border town from Islamic State militants, Turkey's prime minister's office said.

The state-run Anadolu Agency said the operation, which started hours after Turkey indicated it would step up its engagement in Syria, began at 4 a.m. with Turkish artillery launching intense fire on Jarablus from the Turkish town of Karkamis, followed by Turkish warplanes bombing IS targets in the town.

Haber Turk television and Hurriyet newspaper, citing unnamed military sources, said a ground offensive has not started. Hurriyet said efforts were underway to open a "passage" into the region. It quoted the sources as saying Turkish Howitzers and rocket launchers had fired 224 rounds at 63 targets within an hour and 45 minutes, and that the Turkish air raids started just after 6 a.m.

The Anadolu Agency said the operation aims to clear Turkey's border of "terror organizations" and increase border security. It said the aim also is to "prioritize and support" Syria's territorial integrity, prevent a new refugee wave and provide humanitarian aid in the region.

The operation was launched hours before U.S. Vice President Joe Biden was due in Ankara for talks that include developments in Syria.

The prime minister's office said a border area in Turkey had been declared a s"pecial security zone," and asked journalists not to try access it, citing safety concerns and threats posed by the IS.

On Tuesday, Turkish Foreign Minister Mevlet Cavusolgu pledged "every kind" of support for operations against IS along a 100-kilometer (62-mile) stretch of Syrian frontier, putting the NATO member on track for a confrontation with U.S.-backed Kurdish fighters in Syria, who have been the most effective force against IS and who are eyeing the same territory.

"It is important that the terror organizations are cleansed from the region," Cavusolgu said in a joint news conference with his Hungarian counterpart.

The Britain-based Syrian Observatory for Human Rights monitoring group said some 500 Syrian rebels were massed on the Turkish side of the border in preparation for an assault, including local fighters from Jarablus. One rebel at the border told the BBC the number was as high as 1,500 fighters.

The latest developments have thrust the town into the spotlight of the ongoing Syrian civil war. Jarablus, which lies on the western bank of the Euphrates River where it crosses from Turkey into Syria, is one of the last important IS-held towns standing between Kurdish-controlled areas in northern Syria.

Located 20 miles (33 kilometers) from the town of Manbij, which was liberated from IS by Kurdish-led forces earlier this month, taking control of Jarablus and the IS-held town of al-Bab to the south would be a significant step toward linking up border areas under Kurdish control east and west of the Euphrates River.

Turkey had increased security measures on its border with Syria, deploying tanks and armored personnel carriers in recent days. On Tuesday, residents of the Turkish town of Karkamis, across the border from Jarablus, were told to evacuate after mortars believed to be fired by IS militants landed there.

Turkey has vowed to fight IS militants at home and to "cleanse" the group from its borders after a weekend suicide bombing at a Kurdish wedding in southern Turkey killed at least 54 people, many of them children. Turkish officials have blamed IS for the attack.

Ankara is also concerned about the growing power of U.S.-backed Syrian Kurdish forces, who it says are linked to Kurdish groups waging an insurgency in southeastern Turkey.

The Kurdish-led group known as the Syria Democratic Forces, or SDF, recaptured Manbij from IS earlier this month, triggering concerns in Ankara that Kurdish forces would seize the entire border strip with Turkey. The U.S. says it has embedded some 300 special forces with the SDF, and British special forces have also been spotted advising the group.

Syrian activists, meanwhile, said that hundreds of Turkish-backed Syrian opposition fighters were gathered in the Turkish border area near Karkamis in preparation for an attack on Jarablus.

Nasser Haj Mansour, an SDF official on the Syrian side of the border, said the fighters gathering in Turkey include "terrorists" as well as Turkish special forces. He declined to comment on whether the SDF would send fighters to the town, but an SDF statement said the Syrian Kurdish force was "prepared to defend the country against any plans for a direct or indirect occupation."

The reports and rhetoric appeared to set up a confrontation between the SDF, the most effective U.S. proxy in Syria, and NATO ally Turkey.

A rebel commander affiliated with the SDF was killed shortly after broadcasting a statement announcing the formation of the so-called Jarablus Military Council and vowing to protect civilians in Jarablus from Turkish "aggression."

The Jarablus Military Council blamed the killing on Turkish security agents. There was no immediate comment from Turkey. Haj Mansour said two suspects were in custody but declined to comment on their identities.

___

Issa reported from Beirut. Associated Press writers Zeina Karam in Beirut, Albert Aji in Damascus, Syria, and Nataliya Vasilyeva in Moscow contributed to this report.