NEW DELHI, Feb. 16 (Xinhua) -- At least seven workers were killed and one felt unconscious Friday due to asphyxiation while entering a drain in southern Indian state of Andhra Pradesh, police said.

The incident took place at village Moram in Chittoor district about 534 km south of Amaravathi city, the capital of Andhra Pradesh.

"Today in a hatchery here seven workers were killed while entering a drain with the purpose to clean it," Choudeswari, a senior police official at Chittoor told Xinhua.

"Initially four workers went inside and fell unconscious. However, after getting no clue from inside, four others who entered the drain to rescue them also fell unconscious."

Following the incident, residents raised an alarm and informed the police, who rushed rescue teams to the spot.

"The rescue teams brought all the eight out, however, four were declared dead at the hospital, three others succumbed while undergoing treatment," the police official said.

"The deaths have taken place due to asphyxiation after inhaling poisonous gas inside the drain." One person is undergoing treatment, authorities said.

Police have registered a case and ordered an investigation into the incident.

Locals alleged the workers entered the drain without taking precautions or wearing a proper gear after management ordered them to clean the drain.