Iran's health minister said 18 people were killed and 108 wounded in Tuesday night's US strikes across Iran. The Iranian Red Crescent said four people were killed and 67 wounded at a wedding ceremony near the coast of the Strait of Hormuz.
The return of combat operations has alarmed countries throughout the region, as Iran launched missile and drone attacks against what it said were US assets in Bahrain, Jordan, Kuwait and Iraq.
The guns had largely gone silent since the previous exchange in July, although peace talks failed to advance in the interim.
US President Donald Trump threatened to unleash yet more military force, even though the war is unpopular at home and has driven up the cost of gasoline.
Iran, for its part, has remained defiant, despite suffering heavy military and economic losses.
United Nations Secretary-General Antonio Guterres was "deeply alarmed by reports of civilian casualties, including a strike that reportedly hit a wedding ceremony in Iran," and called for an immediate cessation of military action, his spokesman Stephane Dujarric said.
"The secretary -general condemns all attacks against civilians. He recalls that all parties must respect their obligations under international humanitarian law, including the principles of distinction, proportionality, and precaution," Dujarric said.
The deaths would be in addition to the 3636 people documented as killed in Iran as of April 7, including 1701 civilians, according to the Human Rights Activists News Agency (HRANA), an independent Iranian rights group.
The United States has reported 18 military personnel killed and more than 750 wounded.
"We are aware of reports, which originated from Iranian state media. The US military never targets civilians, unlike the IRGC," US Navy Captain Tim Hawkins, a spokesman for the US military's Central Command, said in an email.
The Iranian Red Crescent Society released video showing debris strewn on the ground next to a building with a large hole in the roof, accompanied by still images showing people, some in Red Crescent vests, at a damaged building and a utility tower lying on the ground.
Reuters was able to verify the location using satellite imagery, which showed the fallen tower was 135m from the damaged building.
The wedding incident recalled the blast at a school in Minab that killed 160 people at a girls' school on the first day of the war.
The Pentagon has yet to issue an official finding on the Minab strike, which would be the US military's largest civilian casualty incident in decades. It may have been the result of US use of outdated targeting data, two sources familiar with the matter told Reuters.
Democratic members of the US Congress have called on the administration to release findings of its investigation of the incident, but the report has not been sent to lawmakers.
US defenders of the Iran war also point to civilian deaths that resulted from Iranian attempts to put down street protests that embroiled the country in December and January, before the US and Israel attacked Iran on February 28. HRANA has put that death toll at more than 7000, while Iranian exile groups claim the number is much higher.
Trump has cited an incentive for Iranians to overthrow their government as one reason for the war, in addition to seeking to halt Iran's nuclear program and ending its support for militant groups abroad such as Hamas, Hezbollah and the Houthis.
Iran has also responded to the US attacks by threatening any oil tankers that transit the Strait of Hormuz without Iranian authorisation, severely curtailing traffic in a waterway that before the war carried 20 per cent of the world's oil supply.
