Ancient Persians were the first to use chemical warfare against their enemies, a study has suggested.

A UK researcher said he found evidence that the Persian Empire used poisonous gases on the Roman city of Dura, Eastern Syria, in the 3rd Century AD.

The theory is based on the discovery of remains of about 20 Roman soldiers found at the base of the city wall.

The findings were presented the Archaeological Institute of America's annual meeting.

The study shows that the Persians dug a mine underneath the wall in order to enter the city.

They also ignited bitumen and sulphur crystals to produce dense poisonous gases, suggested Simon James, an archaeologist at the University of Leicester.

He added that underground bellows or chimneys probably helped generate and distribute the deadly fumes.

The Romans apparently responded with counter-mines in an effort to thwart the siege.

"For the Persians to kill 20 men in a space less than 2m high or wide, and about 11m long, required superhuman combat powers - or something more insidious," said Dr James.

"The Roman assault party was unconscious in seconds, dead in minutes."

Excavations showed that the soldiers' bodies were stacked near the counter-mine entrance by the attackers to create a protective barricade before setting the tunnel on fire.

"It is clear from the archaeological evidence at Dura that the Sasanian Persians were as knowledgeable in siege warfare as the Romans," said Dr James.

"They surely knew of this grim tactic."

Evidence also shows that the Persians dug their mine with the intention of collapsing the city wall and adjacent tower.

Although the mine failed to destroy the structures, the attackers eventually conquered the city.

However, how they broke into the city still remains a mystery because details of the siege cannot be found in surviving historical records.

Dura was later abandoned, and its inhabitants were slaughtered or deported to Persia.

In 1920, the well-preserved ruins were unearthed by Indian troops trying to dig defensive trenches along the buried city wall.

The structures were excavated in a series of campaigns in the 1920s and 1930s by French and American researchers.

In recent years, they have been extensively re-examined using modern technology.

Dr James and a colleague are currently investigating records and objects collected about 80 years ago.