Zamboanga City, Philippines - Suspected Muslim militants lobbed a grenade at a late-night concert in a southern Philippine city, wounding 21 people, officials said Wednesday.

The victims were among hundreds of people attending a free concert outside a fast-food restaurant in Isabela City in Basilan province, 900 kilometres south of Manila, on Tuesday evening.

Provincial police chief Salik Macapantar said unidentified men lobbed the grenade at the crowd at the height of the concert.

"It was chaos," he said. "The wounded were rushed to nearby hospitals."

Brigadier General Rustico Guerrero, provincial marine commander, said authorities suspect that al-Qaeda-linked Muslim Abu Sayyaf rebels were behind the attack.

He said the attack could be in retaliation for the military's offensives against the rebels.

"There is an ongoing operation against the terrorists and this could be a diversionary tactic," he said.

Basilan province is one of the strongholds of the Abu Sayyaf, which is the smallest but most violent Muslim rebel group in the southern Philippines.

It has been blamed for some of the worst terrorist attacks in the Philippines, as well as a number of high-profile kidnapping-for-ransom cases. (dpa)