Trump announced Tuesday that "a whole civilization will die" if Iran does not heed his ultimatum to accept US war demands, while Vice President JD Vance warned that Washington had additional "tools in our toolkit" that could be deployed against Tehran.

- New 'tools' -

The United States has already used a wide variety of military capabilities in the conflict to strike thousands of targets in Iran.

"The US has employed a significant number of its most advanced capabilities in the war -- air assets, cruise missiles, advanced stealth bombers, one-way attack drones," said Daniel Schneiderman, director of global policy programs at Penn Washington, the University of Pennsylvania's center in the nation's capital.

There may be "exquisite unique hypersonic capabilities or other bespoke systems that could be used against specific targets," he said.

But "unless we're talking about nuclear weapons, I don't think the administration has limited itself in terms of what it is employing."

US officials have also refused to rule out the deployment of ground troops -- a step that would mark a major escalation in the war.

Etienne Marcuz, associate researcher at French think-tank FRS, said that "among the things they can still do, there is the possibility of cyber" warfare.

Trump has said US forces employed a weapon he referred to as the "discombobulator" during the January raid to capture Venezuelan leader Nicolas Maduro that helped disable the country's military equipment.

- Nuclear option 'unlikely' -

Trump's threat to wipe out Iranian civilization has sparked speculation that nuclear weapons could be employed in Iran. In addition to massive city-destroying strategic warheads, Washington also possesses smaller "tactical" weapons designed to be used on the battlefield.

The United States is the only country to have used nuclear weapons in combat -- against Japan late in World War II -- and Trump ordered a resumption of nuclear testing last year.

The White House pushed back Tuesday against suggestions that Washington could target Iran with nuclear weapons, saying that "literally nothing" Vance said implied it would do so.

Schneiderman said it is "highly unlikely that the US will choose to use nuclear weapons against Iran. It is the ultimate Rubicon to cross."

He noted that consequences of a nuclear strike include "the loss of life and human suffering that would be unleashed, the global economic turmoil and environmental impacts of nuclear fallout circulating in the atmosphere, and the rendering of Iran's oil and natural gas exports functionally unusable."

Marcuz agreed, saying the "political cost of such a deployment would be enormous," and that it could "pave the way for similar action by Russia in Ukraine."

-What will US do? -

Trump has already threatened that the United States could bomb Iran's bridges, power plants and other civilian infrastructure into the "stone age."

If Trump's ultimatum expires Tuesday without a deal, "there's a possibility the administration escalates their attacks on dual-use infrastructure like bridges and power and energy systems," Schneiderman said.

"The risk of lasting damage to Iran's infrastructure and the immiseration of the population above and beyond what they're already experiencing is significant," he said.

"The likelihood that these strikes achieve a strategic effect on the course of the war is minimal, and the likelihood they harm the civilian population is high."