JOHANNESBURG (AP) — Over 500 people in South Africa have fallen ill from a gastrointestinal illness that causes stomach cramps and diarrhea, which authorities are blaming on poor water quality in two different cities in the country.

Authorities this week issued a warning to communities east of the capital, Pretoria, to stop drinking water from their taps after 408 people including children fell ill and were treated at various health facilities.

Like other cities in the country, the capital and its suburbs have long had issues with the cleanliness of the region’s water, and it remains a contentious political issue.

On Wednesday, city officials were distributing water to residents through emergency tankers, while the charity organization Gift of the Givers supplied the community with bottled water.

Most of those who fell ill were children under 4. According to health and city officials, the people fell ill after drinking contaminated water from a water treatment plant which supplies water to several communities east of Pretoria.

Authorities confirmed that the water at the plant had not been chlorinated for about three weeks, rendering it unsafe for human consumption. Officials said the municipality had trouble obtaining chlorine from its usual supplier.

They said the plant gets water from the Bronkhorstspruit River, which is badly contaminated with sewage pollution from poorly functioning wastewater treatment plants upstream.

An initial investigation revealed that officials failed to inform residents about the poor quality of the water.

Nasiphi Moya, mayor of the city of Tshwane, which includes Pretoria and surrounding communities, said that an investigation found that city officials had failed to warn affected areas “that the water being supplied had not been chlorinated and therefore should not be drunk or used for cooking, washing or bathing, even after boiling.”

Various political parties have called for Moya to resign, and for other officials to be fired, to take responsibility for the water contamination.

In the coastal city of Gqeberha, in the Eastern Cape province, at least 111 students from Rhodes University have been treated for a gastrointestinal illness over the last few days, the university said Wednesday.

University and water authorities were investigating the cause of the outbreak, but they also suspected it was due to the water quality.

“Students who experienced severe symptoms were taken to hospital for medical attention and have since returned to campus,” university spokesperson Olwetu Gwanya said on Wednesday.

Staff and students were urged to remain vigilant and practice safe water use, food handling and personal hygiene, both on and off campus.

“These measures include drinking boiled and cooled or bottled water, using safe water to prepare food and brush teeth, avoiding ice and uncooked food washed in untreated water and washing hands regularly with soap and water,” Gwanya said.