US President Donald Trump on Saturday threatened “bad things” would happen to Afghanistan if it does not give back control of the Bagram air base to the United States, and declined to rule out sending in troops to retake it.

“If Afghanistan doesn’t give Bagram Airbase back to those that built it, the United States of America, BAD THINGS ARE GOING TO HAPPEN,” Trump said in a Truth Social post.

Trump said on Thursday that the United States had sought to regain control of the base used by American forces following the attacks of September 11, 2001. He told reporters on Friday that he was speaking with Afghanistan about it.

The withdrawal of American forces in 2021 led to a takeover of US bases, and the toppling of the US-backed government in Kabul, by the Islamist Taliban movement.

Afghan officials have expressed opposition to a revived US presence.

Current and former US officials privately caution that re-occupying Bagram air base in Afghanistan might end up looking like a re-invasion of the country, requiring more than 10,000 troops as well as deployment of advanced air defenses.

Trump, who has previously said he wants the United States to acquire territories and sites ranging from the Panama Canal to Greenland, has appeared focused on Bagram for years.

Asked on Saturday whether he would send in US troops to retake the base, Trump declined to give a direct answer, saying: “We won’t talk about that.”

“We’re talking now to Afghanistan and we want it back and we want it back soon, right away. And if they don’t do it - if they don’t do it, you’re going to find out what I’m gonna do,” he told reporters at the White House.

The sprawling airfield was the main base for American forces in Afghanistan during the two decades of war that followed the September 11, 2001, attacks in New York and Washington by al Qaeda.

The base once counted fast-food restaurants like Burger King and Pizza Hut catering to US troops as well as shops selling everything from electronics to Afghan rugs. It also hosted a massive prison complex.

Experts say the sprawling air base would be difficult to secure initially and would require massive manpower to operate and protect.

Even if the Taliban accepted the US re-occupation of Bagram following negotiations, it would need to be defended from a host of threats including Islamic State and al Qaeda militants inside Afghanistan.

It could also be vulnerable to an advanced missile threat from Iran, which attacked a major US air base in Qatar in June after the United States struck Iranian nuclear sites.