Speaking at a White House briefing on Thursday, Vance said he was "extremely sceptical" of reports from Iranian state media about the strike, which hit a family wedding ceremony in the coastal town of Kuhestak on Tuesday.
"I will say that the Iranian state media has not been a very good scribe about what's happened in the conflict thus far," Vance said. "I know that we're investigating."
The Iranian Red Crescent Society said shrapnel from a missile struck the ceremony at a private home, killing at least four people — named locally as Zarkhatoon Taheri, 50; Kolsum Mallahi Nazhdnia, 43; Mohammad Mallahi, 16; and Amirali Karimi, four. Iranian state media later reported a fifth death, a 22-year-old woman who died in hospital.
Iranian Foreign Minister Esmaeil Baqaei described the attack on X as a "war crime" and vowed that "Iran will respond to these savage crimes with firmness." Ebrahim Azizi, chair of the Iranian parliament's national security committee, added: "Make no mistake: these crimes will not go unpunished."
US Central Command said it had carried out strikes targeting Iran's Revolutionary Guards after their "recent attempted attacks … against commercial shipping in the Strait of Hormuz and against American service members deployed to the region." A CENTCOM spokesman insisted "the US military never targets civilians."
Iran has been at war since February 28, when the United States and Israel launched a surprise bombing campaign. An April ceasefire brought an end to the initial phase of intense bombardment, but sporadic exchanges have continued, particularly around the Strait of Hormuz.