Marawi (Philippines) (AFP) - Philippine troops pounded Islamist militants holding parts of southern Marawi city with air strikes and artillery Saturday as more soldiers were deployed and the death toll rose to more than 300 after nearly a month of fighting.

Fires erupted and dark plumes of smoke rose from enclaves still occupied by the militants as the air force staged bombing runs to support ground troops struggling to dislodge the fighters from entrenched positions, AFP journalists at the scene said.

MG520 attack helicopters and FA50 fighter jets were used in the raids, while sustained bursts of automatic gunfire could be heard in the distance, indicating the intensity of the fighting.

Also on Saturday, 400 fresh troops were airlifted to Marawi from the central Philippines, ANC television said quoting military officials.

Television footage showed the soldiers bidding goodbye to their families before being flown to the conflict zone.

Hundreds of militants -- supported by foreign fighters -- rampaged through Marawi, the largely Christian Philippines' most important Muslim city, on May 23 waving black flags of the Islamic State (IS) group.

President Rodrigo Duterte declared martial law in the entire southern region of Mindanao to counter the attack, which he said was part of a plan by IS to establish a base in the country.

Such a base could be crucial for IS' ambitions to establish a caliphate in Southeast Asia, analysts say.

The military has said eight foreign fighters from Chechnya, Yemen, Malaysia and Indonesia were among the militants killed in the Marawi fighting.

- Hundreds of thousands displaced -

The overall death toll rose to 329 with 310 -- 225 militants, 59 soldiers and 26 civilians -- killed in the conflict, according to government figures.

The 19 others deaths came from those displaced by the fighting, said Mujiv Hataman, the governor of a Muslim autonomous region in the south.

Hataman said the deaths among the evacuees were caused by severe dehydration from diarrhoea.

More than 309,000 people have been been displaced in Marawi and nearby areas, the government said. Many have fled to the homes of friends and relatives and others are in evacuation centres.

"Our forces are moving towards the heart of the enemy," regional military spokesman Jo-ar Herrera told reporters in Marawi on Saturday, referring to the heavy fighting under urban conditions.

"It's the centre of gravity. This is where the location of their command and control, the leadership of the enemy."

Ground commanders estimate "more than 100" militants are still holding out in at least four villages in Marawi, military spokesman Brigadier General Restituto Padilla said in Manila.

But he said the figures were based on estimates a few days ago "so this number could have dropped significantly".

Padilla said in an interview with DZMM radio the military would no longer give any self-imposed deadlines on when the militants would be driven out after failing to meet previous ones they had set.

"We are trying our best to expedite (driving them out) without unduly compromising the lives of our soldiers and at the same time the remaining civilians there," he said.