BAGHDAD (AP) — A large explosion hit an arms depot belonging to an Iranian-backed militia faction north of Baghdad on Tuesday, the latest in a series of mysterious blasts at military bases and munitions depots around the country in the past few weeks.

The explosions have occurred in bases and warehouses belonging to militia groups under the umbrella of the mainly Iran-backed militias known as the Popular Mobilization Forces. The state-sanctioned PMF militias have fought alongside Iraq's regular armed forces against the Islamic State group.

Last week, a massive explosion at a munitions depot southwest of Baghdad killed one civilian and wounded 13 others. The exact cause for the blast, which terrified residents and damaged nearby homes, is still unknown although some officials blamed it on faulty storage.

Last month, an explosion took place at a base in Amirli, in Iraq's northern Salaheddin province, killing two Iranians and causing a huge fire. That attack was blamed on an unmanned drone that dropped explosives, with some Shiite militias blaming it on the Islamic State group.

In response to the explosions, Iraqi Prime Minister Adel Abdul-Mahdi last week banned unauthorized flights throughout the country and ordered all military camps and munitions warehouses to be moved outside Iraqi cities. He ordered an urgent investigation into the explosions, whose results he said should be concluded within a week.

No one has claimed responsibility for those explosions or Tuesday's blast near the Balad air base in Salaheddin province, which sent smoke billowing from the area.

The officials who confirmed the explosion spoke to The Associated Press on condition of anonymity in line with regulations.

They said the blast occurred in a depot belonging to the PMF and that an investigation was underway.

The Iraqi Civil Defense said in a statement that the blast occurred near Balad air base, one of the country's largest, which is home to U.S. trainers and is about 80 kilometers (50 miles) north of Baghdad. It said its members were trying to control a fire caused by the blast.

A Shiite militia group is stationed nearby.

The mysterious blasts have given rise to a host of theories, including that Israel may have conducted an airstrike.

Israel has struck Iranian bases in neighboring Syria on numerous occasions, and there has been speculation that it might be expanding its campaign to target Iranian bases to include Iraq.

However, neither the Iraqi government nor Israel have addressed the reports.