WASHINGTON - While saying the United States is "nearing completion" of its core objectives in Iran, US President Donald Trump on Wednesday night threatened to hit Iran "extremely hard over the next two to three weeks," a timeline he has recently set for ending the monthlong war.

"I can say tonight that we are on track to complete all of America's military objectives very shortly. We are going to hit them extremely hard over the next two to three weeks. We're going to bring them back to the Stone Ages where they belong," Trump said in a primetime address to the nation.

The president insisted that the negotiations between the United States and Iran "are ongoing" despite Iran's denial, saying Iran's "regime change has occurred because of all of their original leaders' deaths" and "the new group is less radical and much more reasonable."

"If there is no deal, we are going to hit each and every one of their electric generating plants very hard and probably simultaneously," Trump said. He also signaled the US military could target Iran's oil infrastructure.

Trump again urged US allies to "build up some delayed courage" and take the lead in reopening the Strait of Hormuz, suggesting Washington may end the US-Israeli war with Iran without reopening the crucial global energy waterway, whose prolonged closure has fueled a global energy shock and sent oil and gas prices sharply higher.

"Go to the strait and just take it, protect it, use it for yourselves," Trump urged US allies, claiming again that the strait would "just open up naturally" with the end of the war.

Grappling with market volatility and public concerns over a drawn-out war, Trump argued the ongoing conflict, now in its fifth week, is far shorter than wars such as World War II, Vietnam or Iraq, and should be viewed as a necessary "investment" in the future rather than another prolonged conflict abroad.

Trump used the primetime address to justify the Iran war, tout US military gains, and assure the US public the war is nearing its end, local analysts say.

Markets reacted negatively to Trump's address on his Iran war strategy, with S&P 500 futures falling 0.75 percent, Nasdaq futures down 1 percent, and Dow futures dropping more than 310 points.

Meanwhile, oil prices surged, with US crude rising from about 98 US dollars to nearly 104 dollars a barrel, while Brent crude climbed from around 99 dollars to 106 dollars.

Up to 67 percent of Americans believe that Trump does not have a clear plan for handling the situation in Iran, according to the latest CNN poll.