JERUSALEM, Dec 9 (Maayan Lubell/Reuters) - Israel's army said on Thursday it had used potentially lethal teargas grenades of a type banned by its rules, but promised it would never happen again.

Their use was brought to light originally by a rights group which said in its findings that some units in the West Bank had fired the grenades during pro-Palestinian demonstration.

The group, B'Tselem, said it had documented three incidents in November in which dozens of extended-range teargas grenades were fired at demonstrators in the West Bank village of Nabi Salah.

The army said it was aware of the situation, adding that regulations had now been clarified to ensure there would be no further illicit use of the ammunition.

After investigation, it appears that the soldiers in question used long-range tear gas canisters to disperse a riot, in contradiction with existing IDF regulations, the army spokesman said in a statement.

After the incident, the relevant regulations were clarified and the incident was investigated to ensure that this would not happen again.

A video filmed by B'Tselem on Nov. 12 and obtained by Reuters showed a smoking canister on the ground with the words long range printed on it in Hebrew.

The group said it was filmed at a clash between the army and pro-Palestinian demonstrators.

Heavier than an ordinary teargas canister, the extended range round has a strong propellant jet and is almost impossible to detect in flight, making it extremely dangerous, said Sarit Michaeli, B'Tselem spokeswoman.

What makes them doubly dangerous is the army's practice of firing teargas canisters directly at protesters instead of lobbing them into the air above the action, Michaeli said.

Teargas is used frequently by the army in what have become regular stone-throwing clashes at fixed flashpoints along Israel's West Bank barrier route.

The Israeli Military Judge-Advocate General in July ordered a probe into the death of Basem Abu Rahmeh, who was hit in the chest by a teargas canister that Israeli troops fired during a protest in April 2009 in the West Bank village of Bilin.

A forensic report issued by B'Tselem said the canister which hit Abu Rahmeh in the chest was an extended-range round.