"American evil in the region will be met with heavier, more widespread, and devastating responses, and any country that co-operates with the aggressive American army must accept its dangerous consequences," Iran's military command said.
Jordan's military said it dealt with 13 missiles, 10 of which were intercepted and three which fell in remote areas. Iran said it killed US forces in Jordan, but US officials told Reuters there had not been any casualties, according to initial assessments.
In Kuwait, Iran said it had carried out a missile and drone attack on the vast US Ali Al Salem Air Base and targeted the accommodation of a US commander. Kuwait's fire service said crews put out a blaze at a residential complex on Wednesday that was struck by an Iranian drone causing damage but no casualties.
Qatar, home of the largest US air base in the Gulf, said it "holds the Islamic Republic of Iran fully legally responsible for these attacks and their repercussions and consequences."
Bahrain, which hosts an important US naval base, said its air defence systems intercepted incoming fire from Iran on Wednesday, accusing Iran of "attacks targeting civilians" in a statement issued by its military.
Earlier, state media reported at least 15 were killed following the latest wave of US military strikes on Tuesday.
Seven people were killed and eight others injured in the south-western province of Khuzestan, the state news agency IRNA reported, citing the provincial deputy governor.
The strikes targeted areas around the city of Ahvaz and the town of Aghachari, both located near the Iraqi and Kuwaiti borders.
Iran's Islamic Revolutionary Guard Corps (IRGC) said four air force personnel were killed in the western province of Kermanshah, according to the Tasnim news agency. The province is home to several strategically important missile bases.
Iranian media reported four deaths at a wedding celebration in the south-eastern province of Hormozgan, on the Strait of Hormuz.
The US military's Central Command said it completed a wave of strikes against Islamic Revolutionary Guard Corps targets on Tuesday, including air defence sites, radar systems, maritime assets and facilities, mine-laying capabilities, and communications sites.
Iranian media reported attacks on multiple targets around the country on Tuesday, many near the Strait of Hormuz, where Iran has been threatening oil tankers that dare transit the narrow waterway without authorisation.
The exchange of fire marked the first serious combat since July, ending a period in which the warring parties withheld fire but also showed little interest in returning to negotiations.
The lull came as the conflict appeared to be draining both sides.
Iran's already ailing economy has been battered by war and its conventional military forces have been degraded.
Meanwhile, with US President Donald Trump confronting higher fuel prices ahead of November congressional elections, the US Army has used up much of its global stockpile of highly accurate long-range missiles, raising concerns about the military's readiness for future conflicts.
Nonetheless, the renewal of hostilities produced another exchange of defiant statements.
"I like our position now much better, with almost total control of the Hormuz Strait, and their economy totally collapsing," Trump said.
"They are just playing out the inevitable. When are the Iranian people going to rise up and fight?"
Iran's joint military command responded in kind.
"We warn that the continuation of American evil in the region will be met with heavier, more widespread, and devastating responses, and any country that cooperates with the aggressive American army must accept its dangerous consequences," it said in a statement.
with DPA and AP
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