Iran's foreign ministry said US strikes in Iran's southern Hormozgan province, where Iranian media reported sounds of explosions early on Tuesday, represented a "gross violation" of a tenuous ceasefire in place for nearly seven weeks.
Iran's Revolutionary Guards said they reserved the right to retaliate against any ceasefire violations, while Iran's Supreme Leader Mojtaba Khamenei said countries in the region could no longer be a shield for US bases.
US Secretary of State Marco Rubio said on Tuesday that negotiating a deal with Iran could "take a few days", quashing hopes for an imminent end to the conflict
Both sides had indicated progress on a memorandum of understanding that could halt the conflict and restart shipping through the blockaded Strait of Hormuz, while giving negotiators 60 days to negotiate more complex issues including Iran's nuclear program.
Following strikes against targets that the US said included boats attempting to lay mines and missile launch sites, Rubio told reporters that the Strait of Hormuz has to be open "one way or the other".
Despite a ceasefire in place since early April, US Central Command said on Monday it had carried out fresh strikes designed "to protect our troops from threats posed by Iranian forces."
Iran's Revolutionary Guards said on Tuesday that air defense units had downed a US drone and fired at another drone and a fighter jet which they said had entered Iranian airspace over the Gulf region.
