MANILA (REUTERS) - Philippine troops shelled positions held by a small group of pro-Islamic State in Iraq and Syria (ISIS) militants in southern marshland on Friday (Nov 17), as the military pushed on with a new offensive after the country's biggest urban battle in decades.

The army estimated that 2,000 villagers had been displaced by several days of operations in a region straddling two provinces on the island of Mindanao, as the army went after the Bangsamoro Islamic Freedom Fighters (BIFF), a small and splintered rebel group inspired by ISIS.

The latest operation follows the end last month of what was the Philippines' biggest battle since World War Two, in which troops took five months to crush an alliance of ISIS loyalists, including BIFF fighters, in Marawi City.

The occupation of the city by the militants and their dogged resistance spread alarm in the region about the rise of extremism and radical aspirations to create an ISIS caliphate.

Captain Nap Alcarioto, spokesman for the 6th Infantry Division, said troops were shelling BIFF gunmen in support of ground attacks in an area of marshland between the provinces of Maguindanao and Cotobato, about 170km from Marawi.

"We are still awaiting results of operations," he said.

The army said it was fighting a BIFF faction led by Abu Toraypie, a man allied with the Maute group, the biggest militant group in an alliance that led the Marawi conflict.

Toraypie and some of his men had escaped from Marawi and the army was trying to prevent them from regrouping, the army said.

Military aircraft dropped bombs on another BIFF wing in a town close by.

The BIFF broke away from the separatist Moro Islamic Liberation Front (MILF) a decade ago after becoming disillusioned with a protracted process with the government to grant autonomy to what is the mainly Catholic country's only predominantly Muslim region.

Separately, army spokesman Major-General Restituto Padilla said Marawi was clear of militants who had been hiding in the ruins of the pummelled city, but unexploded munitions and booby traps had yet to be cleared.

"The last firefight we had was on Nov 5, when we killed nine terrorists," he told a news conference, adding that all top militant leaders had been killed, although this was subject to DNA confirmation.