(CNN) -- As many as 400 armed Muslim rebels are holding at least 20 people hostage in a southern Philippine city after coming ashore by boat, authorities said Monday.
Philippine police and armed forces have blockaded the areas of Zamboanga City, a mainly Christian city on the southern coast of Mindanao, where the rebels are holed up, Mayor Isabelle Climaco-Salazar told CNN. She said the rebels had planned to march on the city hall.
City officials hope to talk to the rebels, who are believed to be from the Moro National Liberation Front, to try to negotiate the hostages' release, she said.
The MNLF, a separatist movement founded in 1971 by Nur Misuari with the aim of establishing an autonomous region for Muslims in this mainly Catholic country, signed a peace deal with the central government in Manila in 1996 -- though some of its members have broken away to continue with a violent campaign.
According to a statement issued by Climaco-Salazar, the current crisis began at around 4.30 a.m. local time when government forces clashed with armed rebels heading for the city by boat. Six people, including a policeman, a navy serviceman and four civilians, were killed during the course of these clashes.
The statement added that six districts in this coastal city of 800,000 people have been affected by the "infiltration of alleged MNLF members," with 20 hostages taken in the district of Santa Catalina.
In a televised press conference Monday, a police spokesman said a further 200 people were currently stranded in the city because of the blockade by security forces. They are not being classified as hostages.
No demands have been made by the rebels, he added.
Under the terms of the 1996 agreement, Misuari was named as governor of an expanded Autonomous Region in Muslim Mindanao (ARMM). He served until 2001.
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 an another insurgent group, the Moro Islamic Liberation Front (MILF), which has fought for decades to set up an independent Islamic state on the resource-rich island of Mindanao.
CNN's Kathy Quiano contributed to this report.