The slayings took place on a main street in al-Bakir district in the town of Hit.
Anbar provincial council chairman, Sabah Karhout, said the Sunnis killed were tribal fighters allied with the government and members of the security forces captured when the Islamic State group overran the town, located about 140 kilometers (85 miles) west of the Iraqi capital.
Residents said the 30 men were paraded through the city as militants shouted through the loudspeakers that the captured men are apostates who fought against them.
Hit was captured by Islamic State militants earlier this month after heavy clashes with the government security forces and tribal militias.
Iraq is in its worst crisis since the 2011 withdrawal of US troops as Sunni militant groups led by the al-Qaida breakaway Islamic State group have seized a third of the country.
In one lightning offensive over the summer, Iraq's US-trained army and security forces melted away as the extremists advanced and captured key cities and towns in country's north.
In Iraq and along with areas in eastern Syria, the militants have declared a self-styled caliphate and imposed their own harsh interpretation of Shariah law. They also have targeted the country's religious minorities, including Christians and others, killing hundreds and forcing hundreds of thousands to leave their homes. AP