Security patrols have been stepped up in the southern Philippines after a nail bomb killed a US soldier and two Filipinos.

The blast happened on Wednesday outside a bar near a military base in the city of Zamboanga, injuring at least 21 people.

A Filipino motorcyclist who was carrying the bomb was among the dead but police do not think it was a suicide attack.

No-one has claimed responsibility but police have put the Abu Sayyaf kidnap-for-ransom group at the top of the suspects list.

The Abu Sayyaf - a band of armed Muslims that the US links with Osama Bin Laden's al-Qaeda group - last week warned it would mount attacks on civilian, military and US targets.

It has staged bombings in Zamboanga before, but kidnapping is its main activity.

Another home-made bomb went off on the outskirts of the city on Thursday, near a Christian church, but no one was injured.

The American casualties in Wednesday's blast were assigned to a humanitarian mission on Basilan Island, but had been staying at a Philippine army base in Zamboanga.

They remained in the country after most of a US force sent to train local troops to fight Abu Sayyaf rebels ended a six-month mission in July.

'Brutal act'

Wednesday's blast scattered nails around the area, took the roof off a wooden house and damaged at least six nearby shops

The Abu Sayyaf has been linked to al-Qaeda

Police are not ruling out a suicide attack, but they say it is more likely that the bomb went off sooner than intended.

A US embassy spokeswoman in Manila condemned the incident, declaring it a "brutal act of terrorism".

The remains of the dead American are to be flown to a US military base in Okinawa, Japan, a US official said.

As well as the Abu Sayyaf, Communist rebels also operate in the Philippines, and have staged a series of recent attacks.

A military official told the Associated Press that investigators were looking to see if there were any links between the fatal blast and two other incidents on Wednesday.

Local TV reported that a small bomb damaged a parked car near the fence of a police headquarters in the town of Imus, in Cavite province south of Manila.

And police sniffer dogs were called in to inspect several train stations in Manila after concerns that communist rebels might launch an attack.