MANILA (Reuters) - A powerful blast shook a public park in the southern Philippines on Tuesday night, wounding 14 people, a regional police chief said.

Jaime Caringal said the improvised explosive device was left under a wooden bench at Plaza Pershing in downtown Zamboanga City, a port city in the south of the mainly Roman Catholic country.

"The bomb was meant to scare and not to kill," Caringal told reporters, adding the device did not contain shrapnel such as nails or broken glass.

But the blast was powerful enough to shatter glass windows and panels of a shopping mall across the park, named after an American general who had fought in the south in the early 1900s.

Since 2002, hundreds of elite U.S. troops were deployed at a military base in the port city, helping advise and train Filipino soldiers fight Islamic militants.

Philippine security officials said the blast could be the work of a small Islamic militant group with suspected ties to Jemaah Islamiah, a rebel group blamed for the beheading of 10 Marines on the nearby island of Basilan on July 10.

"They are a suspect," Caringal said, referring to the Abu Sayyaf. "We have long been receiving reports of diversionary attacks. We have adequately warned our forces to be vigilant on possible spillover of violence from Basilan and Jolo."

The Philippines launched an offensive against the Abu Sayyaf in early August to punish Muslim rebels blamed for the death of more than 50 soldiers in the two islands since July. Nearly 80 rebels had also been killed in clashes.

On Tuesday, President Gloria Macapagal Arroyo ordered security forces to go on full alert in major urban centres across the country for possible attacks to divert the army's attention from ongoing the offensive on Basilan and Jolo.

The Abu Sayyaf, numbering about 300 fighters, was blamed for the Philippines' worst militant attack, the February 2004 ferry bombing that killed more than 100 people.