The U.N. health agency said it had received multiple reports this week of oil-contaminated rainfall after attacks on energy infrastructure triggered large fires and heavy smoke over the capital, Tehran.

Thick black smoke engulfed parts of Tehran on Monday after an oil refinery was hit in an escalation of strikes targeting Iran’s domestic energy supplies amid the ongoing U.S.-Israeli campaign.

“Black rain and the acidic rain coming with it is indeed a danger for the population, respiratory mainly,” WHO spokesperson Christian Lindmeier told reporters during a briefing in Geneva.

He said Iranian authorities had advised residents to remain indoors due to deteriorating air quality conditions.

Asked whether the WHO supported the recommendation, Lindmeier said the guidance was appropriate given the environmental risks linked to damaged oil infrastructure.

“Given what is at risk right now, the oil storage facilities, the refineries that have been struck, triggering fires and bringing serious air quality concerns, that is definitely a good idea,” he said.

The WHO maintains an office in Iran and works with local authorities to coordinate responses to health emergencies.

A video shared with Reuters by a WHO staff member showed a cleaner mopping up a black liquid at the entrance of the organization’s office in Tehran on March 8, which the staff member said was residue from the reported oil-laden rainfall.

The agency said it continues to monitor the situation and maintain contact with Iranian health authorities as reports of environmental fallout from the refinery strike emerge.

The warning came from the office of U.N. Secretary-General António Guterres, which said damage to energy and water infrastructure could intensify humanitarian risks as hostilities continue.

Speaking at a press briefing, U.N. spokesperson Stéphane Dujarric said the organization was increasingly alarmed by reports of attacks targeting oil infrastructure.

“We continue to raise the alarm over the humanitarian impact of escalating violence across parts of the Middle East, which is driving rising civilian casualties, damage to civilian infrastructure and growing displacement of people,” Dujarric said.

He added that strikes on oil facilities could have “serious environmental consequences” across the region, with potential immediate effects on air quality, access to safe drinking water and food supplies.

The concerns come after Ministry of Interior Bahrain reported on Sunday that an Iranian drone attack damaged a water desalination plant in Bahrain.

Desalination plants are critical infrastructure for Gulf states, supplying much of the region’s drinking water and supporting economic activity in countries with limited freshwater resources.

Dujarric said the United Nations was urging all parties involved in the conflict to take precautions to protect civilians and essential infrastructure.

“We reiterate again that all possible precautions must be taken to protect civilians from the impact of hostilities and to avoid damage to health facilities, schools, water systems and other essential infrastructure,” Dujarric said.

The warning follows a series of recent attacks on energy installations linked to the ongoing conflict between Israel and Iran. Several oil facilities in Iran were targeted by Israeli strikes on Sunday, while Iran has also launched attacks on oil facilities elsewhere in the region.