MANILA- Local terror group Maute may have acquired their firearms with the help of the illegal drug trade, a military spokesman said Friday.

Armed Forces of the Philippines spokesman Brigadier General Restituto Padilla said the rebel group's firearms were acquired over time with the help of drug money.

"It has taken time. In the course of time, nakapag-ipon ito. Naniniwala kami na ang ibang armas na ito ay nanggaling sa pagnenegosyo ng droga," he told DZMM.

Padilla added that support structures of the group have also pooled funds from illegal activities, which included narcotics. Stamping out the drug trade is among the Duterte administration's priorities.

"May iba po silang support structures na ang pinangggalingan ng kanilang tulong ay galling sa mga illegal na activity kasama na ang droga," he said, noting that soldiers have recovered drug paraphernalia in previous operations against the Maute group.

Clashes between government forces and Maute terrorists entered its fourth day Friday after the fighting initially erupted on Tuesday.

The firefight has resulted in the death of 31 terrorists, 11 soldiers, 2 police officers and at least 5 civilians, authorities said.

Speaking to ANC on Friday, Philippine Army 1st Infantry Battalion spokesperson Lt. Col Jo-Ar Herrera said clearing operations in Marawi City were still ongoing.

"We need to push hard. We need to flush them out. We are optimizing all our strengths," he said.

Herrera added that the military was still going after around 20-30 Maute terrorists in the city.

President Rodrigo Duterte on Tuesday placed Mindanao under martial law citing the "rebellion" committed by the terrorists when it raised Islamic State flags around the city.