The US has been advising the Philippines military
US military advisors have made a number of trips to the Philippines to train troops in their hunt for the Muslim separatist group Abu Sayyaf.
News of the latest US visit - described by a military spokesman as "sensitive" - came amid rising expectations that the Philippine military was about to mount a mission to rescue two US hostages and a Filipino held by the rebels.
Martian and Gracia Burnham, Christian missionaries from Witchita, Kansas, were snatched from a western Philippines tourist resort in May along with about 17 Filipinos who have since been killed or released for ransom.
The Burnhams were taken to the Abu Sayyaf stronghold of Basilan island, where they are being held with nurse Deborah Yap.
A third American captured with them, Guillermo Sobero from California, was found beheaded.
'Zeroed in'
The Abu Sayyaf says it is fighting for a separate Islamic state, but kidnapping has been its main activity.
An unspecified number of US soldiers arrived by cargo plane at Edwin Andrews air base, close to Zamboanga city, on Friday. They are members of "Special Operations Command" of the Hawaii-based US Pacific Command, said Lieutenant Colonel Danilo Servando.
"We can't say what their purpose is," he said.
Two groups of US military advisers made preliminary visits to Zamboanga city and Basilan island earlier this month and in October to assess barracks and training facilities.
On Thursday a senior Philippine military commander said troops were closing in on the Abu Sayyaf faction holding the Burnhams and hoped to rescue the Americans in a few days.
"We have zeroed in on our objective. We hope and pray we will really attain this objective," said Lieutenant-General Roy Cimatu.
Soldiers pursuing the rebels have tied yellow ribbons around the their guns, using the well-known symbol of homecoming
"This is a reminder to our soldiers about the necessity of conducting the safe rescue of the hostages so that they can all go back to their families come Christmas," said the general.