Speaking at a Pentagon ceremony marking the 18th anniversary of the terrorist attacks that killed nearly 3,000 people, Trump said that over "the last four days" US forces have "hit our enemy harder than they have ever been hit before and that will continue."
Trump said the assault was ordered after he canceled peace talks with the Taliban over the weekend in retaliation for a bomb attack that killed one US soldier last week.
The precise nature of the US offensive against the Taliban that Trump described was not immediately clear.
On Monday, Trump had already declared that "over the last four days, we have been hitting our Enemy harder than at any time in the last ten years!"
But US troop levels in Afghanistan are only around 14,000, a fraction of the peak of about 100,000 in 2010.
In his Pentagon speech, Trump also issued a threat against militants ever attacking on US soil again, saying the response would be unlike any ever seen before.
"If for any reason, they come back to our country, we will go wherever they are, and use power, the likes of which the United States has never used before," he said.
"I'm not even talking about nuclear power. They will never have seen anything like what will happen to them," he added.
The warlike comments were all the more startling because it was only on Saturday that the Republican former businessman announced on Twitter that he'd been about to meet with Taliban leaders on Sunday at his Camp David presidential retreat.
Before the tweet, no one outside Trump's immediate circle was aware of the development.
It came after months of painstaking, mostly behind-the-scenes negotiations on cutting back the US troop presence and extricating the United States from a long, fruitless war.
It was also stunning for the choice of the prestigious Camp David setting on a date so close to the September 11, 2001, anniversary.
Trump's abrupt reversal of that plan and decision to punish the Taliban for last week's bomb attack was followed by the sacking of his controversial national security advisor John Bolton on Tuesday.