Five US marines have been detained in the Philippines, accused of raping a local woman.
The woman, 22, was allegedly raped by troops who were in the Philippines for joint military exercises.
Foreign Secretary Alberto Romulo said he was "deeply concerned" about the claims, and the US embassy in Manila said it took the report very seriously.
Analysts say the incident could fuel local opposition to America's military presence in the Philippines.
US troops often take part in counter-terrorism training in the country, together with Filipino soldiers.
According to the French news agency AFP, the Subic Bay authority said the alleged victim had been visiting a karaoke bar when she met the five men, on 1 November.
They had then reportedly invited her to get into a rented van with them.
A few hours later, witnesses said they saw the woman being dumped from the van into a road.
"The perpetrators of this heinous crime shall be brought to justice," said Philippine Foreign Secretary Alberto Romulo.
"The US takes reports of violations of US and Philippine law by US military personnel very seriously, and will fully co-operate with the Philippine authorities in the investigation of this incident," the US embassy in Manila said in a statement.
According to press reports, the Philippine authorities briefly delayed the departure of a US warship while searching for the marines.
The five men were then prevented from leaving the country with the rest of their compatriots on Wednesday, and are currently in the custody of the US embassy in Manila.