A small explosion rocked a wharf where US soldiers were unloading supplies from a boat in the southern Philippines, local military officials said on Tuesday.
However, no one was hurt in the incident in Zamboanga city late Monday and it had not yet been determined if what appeared to be a small bomb was targeting the Americans, said regional military spokesman Major Ramon David Hontiveros.
"There is an ongoing investigation into the blast. Soldiers found no shrapnel in the area and we still don't know who was behind it," Hontiveros said.
The lack of shrapnel indicated that the blast was probably caused by a crude home-made bomb, the major said. The explosion did not do any serious damage to the wharf.
The explosion took place near a local military detachment and beside a closed vendor's stall, according to Hontiveros, but he did not say how close the Americans were to the bomb or how many soldiers were there.
The Americans were in the southern port city as part of US efforts to train local troops in fighting Muslim extremists.
US troops have been rotating in the southern Philippines since 2003 to train their Filipino counterparts against the Abu Sayyaf, a small gang of militants with links to Al-Qaeda and blamed for the country's worst terrorist attacks.
The Zamboanga wharf was tightly secured before Monday's explosion, said city police chief Inspector Usman Pingay, adding they were investigating the incident.
The US embassy in Manila had no immediate comment on the incident.