NEW YORK - The dollar rose sharply from two straight sessions of losses on Thursday (April 2) after US President Donald Trump's speech on Iran undermined market expectations of a swift end to the conflict, renewing a bid for safe-haven assets.

Trump vowed more aggressive strikes on Iran in the next two to three weeks during his televised speech on Wednesday, offering no concrete timeline to open the Strait of Hormuz or end a war that has rattled investors and roiled markets.

Iran's military responded with a warning for the US and Israel of "more crushing, broader and more destructive" attacks in store.

The US dollar rose, even against other safe-haven currencies including the Swiss franc and the Japanese yen.

The dollar strengthened 0.6 per cent to 0.799 against the Swiss franc.

Against the Japanese yen, the dollar was up 0.5 per cent at 159.57, nearing the psychologically important 160 level that sparks investor worries of intervention by Japanese authorities.

"In the last couple of days there was a bit of optimism that the war was going to end soon and President Trump's address to the nation yesterday sort of undermined that hope," said Marc Chandler, chief market strategist at Bannockburn Global Forex in New York.