Gunmen killed five policemen in a checkpoint shooting north of Baghdad on Monday, the deadliest of attacks that left 13 dead, officials said amid a nationwide surge in violence. The unrest came days after an Al-Qaeda-linked group claimed responsibility for a spate of car bombings in the capital last week, killing 50, the latest in a spike in bloodshed that has raised fears Iraq is slipping back into all-out sectarian war. In Monday\'s worst attack, gunmen opened fire on a police checkpoint near Tikrit, capital of Salaheddin province, killing the five officers. Also in Salaheddin, two near simultaneous roadside bombs targeted an army patrol along a main road near the Baiji oil refinery, killing two soldiers and wounding three. Six people died in violence elsewhere, among them an alleged militant linked to Al-Qaeda and two policemen, in attacks that included a suicide car bombing. Iraq has seen a surge in unrest since the beginning of 2013, in which nearly 4,000 people have died so far, according to an AFP tally. Officials have vowed to press on with a campaign targeting militants they say has led to the capture of hundreds of alleged fighters and the killing of dozens more. But the government has faced criticism for not doing more to tackle anger in the Sunni Arab community over alleged ill treatment at the hands of the Shiite-led authorities.