Video showing injuries consistent with the use of white phosphorus shells has been filmed inside hospitals treating Palestinian wounded in Gaza City.

Contact with the shell remnants causes severe burns, sometimes burning the skin to the bone, consistent with descriptions by Ahmed Almi, an Egyptian doctor at the al-Nasser hospital in Khan Younis.

Almi said the entire body of one victim was burned within an hour. It was the first time he had seen the effects of what he called a "chemical weapon".

The Israeli military has denied using white phosphorus during the assault on Gaza, but aid agencies say they have no doubt it has been used.

"It is an absolute certainty," said Marc Garlasco, a senior military analyst at Human Rights Watch. He had seen Israeli artillery fire white phosphorus shells at Gaza City, Garlasco said.

The shells burst in the air, billowing white smoke before dropping the phosphorus shell.

Garlasco said each shell contains more than 100 incendiary rounds, which ignite and pump out smoke for about 10 minutes.

Severe respiratory problems can result in anyone exposed to the smoke and burning chemical particles that rain down over an area the size of a football pitch.

According to the International Solidarity Movement, many patients at the hospital near Khan Younis were suffering from serious breathing difficulties after inhaling smoke.

Human Rights Watch compares the use of white phosphorus shells over Gaza to the impact of cluster munitions, which scatter "bomblets" over a wide area. Children may kick and play with a lump of phosphorus, stirring up the embers and producing more fire and smoke.

The use of white phosphorus as a weapon – as opposed to its use as an obscurant and infrared blocking smoke screen – is banned by the UN's third convention on conventional weapons, which covers the use of incendiary devices. Though Israel is not a signatory to the convention, its military manuals reflect the convention's restrictions on using white phosphorus.

Israel initially claimed that it was not using white phosphorus. It later explained that shells being loaded for a howitzer, identified from photographs as phosphorus rounds, were empty "quiet" shells used for target marking. However, images of exploding shells and showering burning fragments are now acknowledged by independent observers as having been phosphorus.

At the centre of the controversy is the way white phosphorus air burst shells have been used in heavily built-up urban areas, with an overwhelmingly civilian population.

The M825A1 rounds, which are the kind identified as being fired by Israeli forces, are made primarily for use as a smokescreen in a way that limits their effect as an incendiary weapon, experts say.

Neil Gibson, a technical adviser to Jane's Missiles and Rockets magazine, said the shells did not produce high-velocity burning fragments like conventional white phosphorus weapons once did.

Instead, he said, they produced a "series of large slower burning wedges which fall from the sky". The wedges would then ignite spontaneously in the air and fall to the ground, burning for five or 10 minutes, he said.