Seven Philippine soldiers were killed and 21 were wounded in fierce clashes with Al Qaeda-linked Islamic militants in the south of the country on Thursday, the military said.
Philippine marines fought a five-hour battle with members of the Abu Sayyaf group in one of the militants' hideouts in the forests of Jolo island, local military spokesman Lieutenant Colonel Randolf Cabangbang told reporters.
"The encounter was fierce and troops encountered a big group of the Abu Sayyaf," he said.
Extra ground troops and two helicopter gunships were called into the area to cover the helicopter evacuation of the wounded to a military hospital on the island, and to track down the attackers, according to Cabangbang.
He said it appeared the soldiers stumbled into a major Abu Sayyaf camp as they chased a small group of the militants on Jolo, which is roughly 11 times the size of Hong Kong island with a rugged and largely lawless interior.
"I think what happened was, the troops were on manoeuvres during the entire night, and in the morning that's when they realised they were already in the (Abu Sayyaf) camp. That explains why we have so many casualties," he said.
He said the soldiers were initially trying to hunt down militants wanted for a series of kidnappings in the area in recent months, including the abductions of an Indian man visiting his Filipina wife's hometown and a Malaysian trader.
Cabangbang said it was believed the militants that the soldiers clashed with were under the control of senior Abu Sayyaf figures wanted for previous kidnappings and killings of foreigners as well as Filipinos in the south.
One of those leaders is Isnilon Hapilon, the subject of a $5-million-dollar reward from the US government for information leading to his capture.
Another is Radullan Sahiron, an ageing Abu Sayyaf figure who lost his right hand fighting security forces in the 1970s and has a $1-million-dollar bounty on his head.
However Cabangbang said it was not clear whether Hapilon and Sahiron were directly involved in Thursday's clashes.
The military was also unable to say if there were any Abu Sayyaf casualties.
The Abu Sayyaf, a small gang of self-styled Islamist militants founded in the 1990s with seed money from Osama bin Laden's Al-Qaeda network, is blamed for the country's worst terrorist attacks.
These include the bombing of a passenger ferry in Manila Bay in 2004 that killed more than 100 people, as well as a string of high-profile abductions and murders in the south.
Hundreds of US troops have been deployed on Jolo and other parts of the southern Philippines since 2002 to help eliminate the Abu Sayyaf.
However they are only allowed to train Filipino soldiers, and not engage in any combat operations.
The Philippine and US militaries have described their joint operations in the south as a success, saying the Abu Sayyaf threat has diminished, with militant numbers down to a few hundred.
However incidents such as Thursday's clashes show the Abu Sayyaf, which the military says at times shelters with relatives or other armed groups in Jolo's Muslim-majority population, remains able to conduct deadly operations.
In Manila, armed forces spokesman Commodore Miguel Jose Rodriguez insisted that, despite the heavy military casualties, Thursday's operation was an important step in quashing the Abu Sayyaf.
He said the military had achieved a "strategic victory" in over-running the camp, even though there were no reports of any Abu Sayyaf militants being captured or killed.