Iranian media and officials have said at least three people have been killed, including a police officer and a protester, accusing "opportunists" and "rioters" of shooting at demonstrators and security forces.
State television said a fourth person was killed on Thursday and two wounded during "rioting" in the western province of Lorestan, where it said people had taken to the streets "on the pretext of the water problems in Khuzestan".
UN rights chief Michelle Bachelet on Friday told Iran to address the chronic water shortage in Khuzestan instead of cracking down on protesters.
"Shooting and arresting people will simply add to the anger and desperation," she said, adding that the "catastrophic" situation had been building up for many years.
Iran's foreign ministry spokesman Saeed Khatibzadeh said Saturday that Bachelet's comments "on the recent events in Khuzestan (are) regrettable", decrying "false accusations and incorrect information".
The rights chief had failed to consider the government's "great efforts" to "relieve the suffering of the population", Khatibzadeh said in a statement, adding that this demonstrated the "political" character of the declaration.
President Hassan Rouhani said Saturday that "despite the action taken, certain problems in Khuzestan province must still be quickly resolved", adding that people had "the right to protest" against the situation.
Human rights groups abroad have accused Iran of using unlawful and excessive force to quell the Khuzestan protests and have cited a higher death toll than that reported by Iranian media.