Security guards of a mining company in western Mongolia clashed with illegal miners known as "ninja miners," leaving many of the combatants injured, local media said Wednesday.

About 50 security guards of a gold mining company fought for over two hours with about 200 ninja miners and local residents in Omnogobi county of Uvs province in western Mongolia.

Many were injured in the fight and three were in critical condition.

The security guards used gas pistols, while ninja miners threw stones and wood against the guards.

The guards called local police, who detained about 17 ninja miners.

It is unclear what caused the clash, and the case is now being investigated by police.

About 100,000 ninja miners dig for gold and other minerals during summer and 30,000 dig throughout the year in Mongolia. Ninja miners include unemployed youths, students, impoverished herders, and others.

Ninja miners are individuals not licensed by the state and mainly look for gold in abandoned gold fields and unauthorized places with hand tools such as shovels and pans.

Periodically, clashes between mining companies and ninja miners break out in licensed gold fields.

A bill to regulate ninja mining activities is pending in the Mongolian parliament.