MARAWI, Philippines — National forces found corpses in the streets of a besieged southern city Sunday, including at least eight civilians who appeared to have been executed, as soldiers battled a weakened but still potent group of militants linked to the Islamic State group. The crisis in Marawi, home to 200,000 people, has grown increasingly dire as the militants show unexpected strength, fending off a military that has unleashed attack helicopters, armored vehicles and scores of soldiers. The violence prompted President Rodrigo Duterte on Tuesday to declare 60 days of martial law in the southern Philippines, where a Muslim separatist rebellion has raged for decades. [...] the recent bloodshed in Marawi has raised fears that extremism is growing as smaller militant groups unify and align themselves with the Islamic State group. Many sent desperate text messages begging to be rescued and reporting that their homes had been destroyed, said Zia Alonto Adiong, an official in Lanao del Sur, one of the country’s poorest provinces. The Associated Press was shown the messages by relief workers at a provincial government complex in Marawi. The violence erupted Tuesday night when the government launched a raid to capture Isnilon Hapilon, who is on Washington’s list of most-wanted terrorists. [...] the operation went awry and militants rampaged through the city, torching buildings and battling government forces in the streets.