A ceasefire has been broken hours after it was agreed, to end a stand-off between the army and separatist fighters in the southern Philippines.

Gunfire has been heard again on Saturday in the southern city of Zamboanga, where Moro National Liberation Front (MNLF) fighters are holed up.

Al Jazeera's Jamela Alindogan, reporting from Zamboanga, said that there was earlier discussion that ceasefire has been agreed between the vice president and MNLF founder, but on the ground there was no letup in the fighting.

"We are hearing firing and mortars. The military, surrounding the area where MLNF fighters are holed up, still has not retreated," she said.

"The defence secretary in fact, confirmed that ceasefire has not been reflected on the ground and it has not been acknowledged at all."

On Friday, the Philippines' vice president said that a Muslim rebel leader had accepted a ceasefire to allow talks on ending a hostage crisis in which his followers had held more than 100 people in a southern port city.

The standoff began on Monday when about 200 fighters from a MNLF rebel faction stormed several coastal communities in Zamboanga city and seized residents.

Twenty-two people, including 15 rebels, have been killed in sporadic clashes between the armed group and troops who have surrounded them, the military said.

Vice President Jejomar Binay had said that rebel leader Nur Misuari had agreed to a truce late on Friday by telephone.

President Benigno Aquino III flew to Zamboanga earlier on Friday to visit government troops and some of the 24,000 residents displaced by the violence.

He warned in a speech that his government would not hesitate to use force to end the most serious security crisis his administration has faced since he came to power in 2010.

Mortar fired

Fighting broke out again in Zamboanga's Santa Catalina village on Friday, and ABS-CBN TV reported that voices presumably of hostages were heard shouting "ceasefire, ceasefire!"

One government soldier was reportedly wounded.

A mortar fired by the rebels landed on a street in front of the government hospital in Santa Catalina.

The MNLF rebels have been overshadowed by a rival group in talks with the government for a new minority Muslim autonomy deal.

Misuari signed a peace deal in 1996, but the armed group did not lay down their arms and later accused the government of reneging on a promise to develop long-neglected Muslim regions in the predominantly Roman Catholic nation.

The government says Misuari kept on stalling and making new demands.

Misuari has not been seen in public since the standoff began.

"There are lines they should not cross," Aquino said of the rebels.

He said the government would use "the force of the state" if those lines are crossed.