Story highlights The helicopters carry out a 20-minute attack on rebel positions in Zamboanga City

The armed forces are trying to "constrict" the rebels' positions, a spokesman says

Authorities estimate that more than 60 people have been killed during the crisis

The unrest in and around the port city has displaced more than 60,000 people

Philippine military helicopters on Monday fired rockets at Muslim rebels who were holed up in areas of a major city on the southern island of Mindanao.

The rebels have been holding scores of people hostage in Zamboanga City for the past week in a crisis that authorities estimate has left as many as 61 people dead and more than 150 wounded.

Intense exchanges of gunfire continued to take place Monday as military officials said they were trying to tighten the noose around rebel-held areas.

Two MG-520 attack helicopters from the Philippine Air Force fired rockets at rebel positions over a roughly 20 minute period in the early afternoon, the official Philippine News Agency reported, citing military officials.

The recent violence has largely paralyzed Zamboanga, a usually bustling trading hub on the southwestern tip of Mindanao, and displaced more than 60,000 people.

The crisis has increased fears of instability in a region where the central government is pursuing a new peace plan after decades of unrest.

President Benigno Aquino III and other top Philippine officials are overseeing the response in Zamboanga, a mainly Christian city.

The Philippine armed forces are carrying out an operation to try to "constrict" the rebels, who came ashore last Monday, said military spokesman Lt. Col. Ramon Zagala.

He described the operation as "delicate," as the troops try to avoid harming hostages and any other civilians caught in the parts of Zamboanga where the rebels are holed up.

The rebels now hold one third of the territory they had last week, before the military began its effort to squeeze their positions last Friday, Zagala said. The armed forces switched to a more offensive approach after the rebels fired mortars into civilian areas, he said.

More than 100 hostages

Military officials say they believe that slightly more than 100 rebels remain active in Zamboanga and that they are still holding more than 100 people as a human shield.

Some hostages and other civilians trapped in the rebel-held zone have managed to flee during outbursts of fighting over the past week.

Security forces estimate that they have killed 51 rebels, although only 21 bodies have been recovered. A further 48 rebels have been captured and nine have been wounded, Zagala said.

The violence has killed six members of the Philippine security forces and four civilians, he said. The number of people estimated to have been wounded, including rebels, stands at 157.

The unrest in Zamboanga has closed schools and businesses. Hundreds of houses of houses have caught fire during the fighting. Authorities have accused the rebels of deliberately starting the fires.

The rebels are a faction of the Moro National Liberation Front (MNLF), a separatist movement founded in 1971 by Nur Misuari with the aim of establishing an autonomous region for Muslims in the mainly Catholic Philippines. The MNLF signed a peace deal with the central government in Manila in 1996, but some of its members have broken away to continue a violent campaign.

Last month, Misuari issued a "declaration of independence" for the Moro nation -- referring to Mindanao's indigenous Muslim population -- after complaining that the MNLF had been left out of a recent wealth-sharing agreement with another insurgent group, the Moro Islamic Liberation Front, which has fought for decades to set up an independent Islamic state on resource-rich Mindanao.