(CNN) -- Philippine police and armed forces negotiated the release of five hostages on Tuesday as a tense stand-off with rebels from the Muslim Moro National Liberation Front entered its second day in Zamboanga City in the southern Philippines.
"This morning five people, including four children, who were held hostage in Talon-talon were released by the rebels," Mayor Isabelle Climaco-Salazar said in a statement, adding that one policeman was wounded in the thigh in an exchange of fire on Tuesday morning.
At least six people were killed yesterday in exchanges of gunfire between the rebels and armed forces.
Authorities said it was unclear how many hostages were still being held by the estimated 300 armed Muslim rebels in the blockaded areas of Zamboanga City, a mainly Christian city on the southern coast of Mindanao.
Philippine Interior and Local Government Secretary Mar Roxas told a press conference that authorities believed as many as 180 people -- which he described as "human shields" -- could be being held hostage.
"These are very soft estimates," said Roxas, who flew in from Manila to supervise operations. Hundreds of troops have been deployed to Zamboanga in response to the crisis.
"It's hard to determine (whether) they're hostages or just residents of the area, but we are taking these numbers quite seriously and that is why the military and the police have exercised restraint."
Roxas did not say whether the rebels had made any demands
"There are ongoing talks, formal and informal, in various stages of maturity," he said. "We are also seeking to validate the bonafides of those presenting themselves as mediators so that we are not led astray in these talks."
An MNLF spokesman told the Philippine Daily Inquirer newspaper that the group was holding 35 hostages.
"We seized them by the roadside so the soldiers won't shoot when they see us," he said.
According to a statement issued by Climaco-Salazar, the crisis began at around 4.30 a.m. local time Monday 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.
She said the rebels had planned to march on the city hall.
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.
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 and Tim Schwarz contributed to this story.