More than 50 elite US-trained scout rangers arrived on Jolo, 1000km south of Manila, aboard two C-130 cargo planes at dawn, military sources said.
The separatist Moro Islamic Liberation Front (MILF) called on government and rebel forces "to refrain from further offensive actions that would lead to more bloodshed and destruction".
"We believe that the declaration of a ceasefire is the most logical step to be taken up by both parties," it said in a statement on Friday.
A senior MILF member, Mohaguer Iqbal, said the group believed it could help to negotiate a ceasefire.
The military said on Friday that nearly 30 soldiers and about 60 fighters had been killed in clashes since Monday. At least 7000 villagers have fled to Jolo town to escape the fighting.
Lieutenant-General Alberto Braganza, the most senior commander in the southern Philippines, said it was beyond his authority to contemplate a ceasefire.
"I have a mission to accomplish on the ground," he said. "I will finish what the rebels had started."
Over the past 24 hours, the fighting has been concentrated in the town of Panamao, where a Philippine officer was killed on Thursday by mortar fire.
Fight goes on
Nearly 4000 soldiers, including several hundred recent reinforcements, have been fighting about 800 fighters from the al-Qaida-linked Abu Sayyaf group and rogue members of the Moro National Liberation Front (MNLF) since a rebel ambush on Monday.
After the MNLF signed a peace deal brokered by Indonesia in 1996, some disaffected followers of former separatist leader Nur Misuari formed alliances with Abu Sayyaf.
The Abu Sayyaf group was
accused of a number of attacks
Misuari's followers reportedly were angered over the killing of four civilians, including a 14-year-old boy, in a military operation against Abu Sayyaf rebels in the town of Maimbung early last week.
The clashes on Jolo are the bloodiest since 2001, when 500 people were killed in a failed uprising led by Misuari, but they are unlikely to affect peace talks with the larger MILF, which broke from the MNLF in 1978.
The MILF, which has its strongholds on Mindanao island, has said it remains committed to the peace process despite calls this week by an Abu Sayyaf leader to rejoin the war for a Muslim state in the south of the mainly Roman Catholic country.
Jesus Dureza, the presidential assistant for Mindanao who has been sent to Jolo, said his mission was limited to overseeing relief efforts for the displaced civilians.
He said a demand by the rebels for Misuari to be moved from a police camp south of Manila to detention on Jolo could only be handled by the courts.
Local Muslim leaders, recalling the burning of most of Jolo town at the height of the secessionist campaign in the 1970s, said they were worried the fighting would escalate.
"We don't want a repeat of the February 1974 incident," said Ulka Ulama, a community leader. "The situation here is very tense. Our national leaders should stop the fighting because the problem might escalate into a full-blown war."
"Our national leaders should stop the fighting because the problem might escalate into a full-blown war"
Despite a superiority in numbers, the military is likely to have trouble boxing in the rebels, who often use warrens of caves and thick jungle to escape after hit-and-run attacks.
Abu Sayyaf and the MNLF renegades have strongholds on Jolo, one of the largest in a string of islands that stretches across the Sulu Sea between the southern Philippines and Borneo island.
Abu Sayyaf, mainly known for kidnappings, has also been accused of several bombings, including one that sank a ferry at the mouth of Manila Bay in February 2004, killing more than 100.