The Red Cross is concerned for the safety of three ICRC hostages
At least six people have been killed in the Philippines, in fighting between the army and Islamic militants who are holding three aid workers hostage.
The troops were engaging the Abu Sayyaf group for the second consecutive day on the southern island of Jolo.
Three Red Cross hostages, two of them foreigners, are being held deep in the jungles of the southern Philippines.
The hostages have now been held for two months, and the Red Cross last heard from them on Wednesday.
Government forces have been trying to box in the militants, to force their surrender, while the hostages have called for talks for their release.
Military spokesman Brigadier-General Gaudencio Pangilinan said the violence had not resulted from an attempt by the troops to rescue the hostages.
It had flared, he said, after rebel gunmen tried to break out of a military cordon now established in the area.
Gang leader Albader Parad may have been wounded or killed in the fighting, the military said.
The three workers from the International Committee of the Red Cross (ICRC) have been held since 16 January in a hilly jungle area near southern Jolo island's Indanan township.
The ICRC said they had last been in touch last Wednesday and that the hostages - Swiss national Andreas Notter, Italian Eugenio Vagni and Filipina Mary Jean Lacaba - were "calm and composed".
They have been enduring constant rain, some illness, and enforced movement as the gunmen holding them sought to avoid military operations.
The Abu Sayyaf has demanded the withdrawal of the military in exchange for freeing the captives.
Military spokesman Brigadier-General Gaudencio Pangilinan told reporters there was no word on the hostages.
"But now we know that they are intact in one group, the Abu Sayyaf and the hostages," he said.
"We learned later that they [the hostages] were nearby" during the clash. "Now they have moved locations, they are being pursued.
"They have moved I think around two or three kilometres from the original encounter site."
AFP news agency reported that troops had found tents and other equipment belonging to the three Red Cross workers.
In a statement, the ICRC said it was concerned that the firefight may have placed the hostages' lives in danger.
Alain Aeschlimann, the ICRC's head of operations in Asia, said that he last spoke with the hostages by phone on 11 March.
"The ICRC is aware that an exchange of gunfire took place in the region," Mr Aeschlimann said. "We're very concerned to hear about this development."