A powerful Kurdish militia said Tuesday that it is mobilizing against al-Qaeda-linked rebels in northeastern Syria after a Kurdish opposition leader was killed in the area, activists and party officials said.

The fight between the Kurds and the extremists has become a war within a war in Syria’s oil-rich region. Recent clashes between Kurdish gunmen and members of al-Qaeda-linked Jabhat al-Nusra and the Islamic State of Iraq and the Levant have left dozens dead on both sides.

The fighting claimed a prominent casualty Tuesday, as a car bomb killed Kurdish leader Issa Hisso, according to the Kurdish Democratic Union Party, the ethnic group’s dominant faction in the region.

“We condemn this ugly criminal act and we promise the martyr and his comrades that we will not stand idle,” the party said in a statement.

Hisso opposed and was imprisoned in the past by President Bashar al-Assad’s regime. He also spoke out against radical Islamist groups, including Jabhat al-Nusra and the Islamic State of Iraq and the Levant. Both groups have gained influence in the Syrian opposition after taking the lead in several battles.

Although no group asserted responsibility for Hisso’s killing, suspicion fell on the al-Qaeda-linked organizations. Nawaf Khalil, a spokesman for the party, said fighters hoped to clear the groups from Kurdish areas.

“The military units have declared mobilization,” he said.

Kurds, the largest ethnic minority in Syria, make up more than 10 percent of the country’s 23 million people. Their loyalties in the conflict are split, though Kurds in opposition areas have carved out a once-unthinkable degree of independence in their areas. They have created their own police force, issued their own license plates and exuberantly gone public with their language and culture.

Also Tuesday, activists reported that an Italian Jesuit priest, Paolo Dall’Oglio, has gone missing while on a trip to the rebel-held north-central city of Raqqah, reportedly to meet with members of the Islamic State of Iraq and the Levant. Dall’Oglio is an Assad opponent who was expelled last year from Syria, where he had lived for 30 years.

It was not clear whether he was still on his mission or had been abducted.

Earlier Tuesday, mortar attacks and air raids in the central city of Homs and the northern town of Andan killed at least 17 people, activists and government officials said.

— Associated Press