WASHINGTON — US forces are using white phosphorus munitions in their fight against the Islamic State — based on pictures and videos posted online by the Pentagon — but it is unclear exactly how the controversial armament is being employed.

White phosphorus shells are intended to make smoke screens or signals for advancing troops. When launched against soldiers and civilians, however, the munition can cause severe burn wounds that can be dangerous for medical personnel treating the injured.

International humanitarian law stipulates that white phosphorus munitions should only be used in areas devoid of civilians. Even using it against enemy combatants has raised concerns, given that the munitions can cause horrific injuries.

Photos posted on a Pentagon-managed public affairs website show a US Army artillery unit in Iraq using white phosphorous munitions, specifically M825A1 155mm rounds. The M825A1 shell can create a smokescreen that lasts about 10 minutes and contains 116 felt wedges impregnated with white phosphorus that jettison and automatically ignite when they come in contact with the air.

On Thursday, Air Force Colonel John Dorrian, the spokesman for the US-led coalition in Iraq, described how he said the munitions were being used.

‘‘In the foreground of the photo are 155mm white phosphorous rounds, which are used for screening, obscuring, and marking. When US forces use these munitions, as required by the Law of Armed Conflict, they do so in a way that fully considers possible incidental effects on civilians and civilian structures,’’ Dorrian said in an e-mail. ‘‘The US military takes all reasonable precautions to minimize the risk of incidental injury to noncombatants and damage to civilian structures.’’

When asked on the phone whether US forces had used white phosphorus munitions for anything other than screening, obscuring, or marking, Dorrian said the munitions had been ‘‘used generally for the circumstances which I described.’’

He could not say how many times it had been used or whether it had been dropped on enemy combatants or their equipment.

The United States has used white phosphorus in Iraq before, notably in the 2004 battle for Fallujah, when Marine artillery batteries were scrutinized for firing the munitions on entrenched insurgents. In Afghanistan, white phosphorus was used by US troops. In 2009, NATO forces there were accused of burning an 8-year-old girl with the munitions.