BAGHDAD (Reuters) - A suicide bomber in a minibus struck a military base in the Iraqi town of Taji on Monday, killing at least 19 people in the latest attack by insurgents trying to undermine the government. It was the third major attack in the last five days and underscored the fragile state of Iraqi security as Washington pulls its remaining 14,500 troops out by the year-end, nearly nine years after the invasion that ousted dictator Saddam Hussein. The attacker detonated a minibus packed with explosives at the entrance to the base, which houses a jail holding al Qaeda, Mehdi Army militia and other prisoners, officials and security sources said. Taji is 20 km (12 miles) north of Baghdad. "Today was the day for the prisoners to meet their families ... at the entrance, there was a minibus driven by a suicide bomber," said a duty officer at the jail. "It entered from the first checkpoint and before the police finished their search this guy drove quickly inside the base and blew it up." Baghdad's security operations centre said the blast killed 19 people, including 11 jail guards, and wounded 24 others. Two other people were missing, security sources said. Violence has dropped sharply since the peak of sectarian slaughter in 2006-07. But Iraqi security forces still struggle to contain daily attacks by Sunni Muslim insurgents tied to al Qaeda and rival Shi'ite Muslim militias. Militants launch scores of bombings and other attacks every month. According to official government figures, 161 civilians were killed in violence in October, the highest toll of the year, along with 97 police and soldiers. Iraqi and U.S. military officials have said Iraq may see an increase in attacks as American troops depart. Soldiers and police are frequent targets. On Saturday attackers struck two areas around the Iraqi capital, killing at least 13 people and wounding more than 20 others. In the southern oil hub of Basra on Thursday three bombs exploded in a busy market, killing 21 and wounding 80. The town of Taji, the site of a major Iraqi military base, was hit by bombers in July, when two blasts in the parking lot of a municipal government building killed at least 28 people and wounded scores of others. On November 14, seven rockets landed in or near the U.S. military's Kalsu base near Iskandariya, 40 km (25 miles) south of Baghdad, wounding two Iraqi civilians living near the base, local police said.