BEIRUT — About 30 Syrian government fighters were killed when rebels set off a bomb in a tunnel beneath a checkpoint in a northwestern province, activists said on Tuesday.Videos and images posted by opposition supporters online showed a massive plume of smoke and earth shooting into the air near a small town as men shouted "Allahu akbar" (God is greatest).Rebels fighting to overthrow President Bashar al-Assad regularly carry out guerrilla attacks against his forces, but the size of the blast, which occurred on Monday, was unusual.The Syrian Observatory for Human Rights, a Britain-based monitoring group, said the blast took place outside the town of Ma'arat al-Nu'man in the northwestern Idlib province.At least two officers were among those killed when insurgents from the Islamic Front and the Shields of the Revolution Council set off tons of explosives in a tunnel running from the road to the checkpoint, the group said.More than 150,000 people have been killed in the three-year-old rebellion, which started as a peaceful protest movement and turned into a civil war after a government crackdown.