President Donald Trump warned Iran on Wednesday that the United States could launch a strike against the fortified Pickaxe Mountain site, urging Tehran not to take actions that could provoke a forceful response. Speaking at a Republican midterm convention, Trump said the United States had observed activity at the site and warned Iran that Washington would respond “very hard” if necessary.

Pickaxe Mountain is located near Iran’s heavily damaged Natanz uranium enrichment complex and contains two deeply buried tunnel systems. The site, situated about 220 kilometers (140 miles) south of Tehran and roughly 2 kilometers from Natanz, has emerged as a potential target amid continued U.S. concerns over Iran’s nuclear infrastructure.

Trump has been threatening military action against Pickaxe Mountain since at least mid-July. The warning comes after the United States and Israel launched attacks against Iran on Feb. 28, prompting Iranian strikes against Israel and Gulf states that host U.S. military bases. Trump has repeatedly identified the degradation of Iran’s nuclear capabilities as a central objective of the conflict.

The nearby Natanz complex, which previously housed two uranium enrichment facilities, has been targeted in multiple conflicts. It was struck in March during the U.S.-Israeli military campaign against Iran and was also hit by Israel during last year’s 12-day war between Israel and Iran. However, the underground tunnel facility being developed at Pickaxe Mountain was not targeted in either conflict, according to the Institute for Science and International Security.

Trump also said he expected the war to end immediately after the U.S. midterm elections, while accusing Tehran of attempting to influence the vote. The elections will determine whether Republicans retain control of Congress, adding a domestic political dimension to the administration’s escalating confrontation with Iran.