Kosovo authorities say they have cut off the water supply to tens of thousands of people in the capital after police arrested five suspects linked to Islamic State who were allegedly planning to poison a reservoir.

Pristina’s water authority said the water supply was shut off early on Saturday “because of security issues”, and that samples were being tested for suspicious substances.

Police say officers patrolling the Badovac reservoir saw three of the men, whose identities have not been revealed, behaving suspiciously. The reservoir supplies almost half of Pristina, a city of more than 200,000 people. Another two suspects were arrested elsewhere in Kosovo.

All face terrorism charges, police said, without giving any further details.

A police source, who spoke on condition of anonymity, told Reuters that two of the suspects had been arrested last year on suspicion of traveling to Syria.

Police have been on alert in recent weeks after Kosovo-born volunteers appeared on Isis propaganda videos warning of imminent attacks against various targets, including water supplies.

Security officials say more than 200 people from Kosovo have gone to fight in Syria and Iraq, and more than 30 are believed to have been killed. There are concerns over the potential threat posed by those who return.