Iran has been fortifying an underground complex near one of its nuclear facilities, according to analysis of new satellite images. This activity comes at a time of heightened tensions as talks between Iranian and US officials continue, with President Donald Trump threatening fresh military action against Iran if it does not agree to a new deal on its nuclear programme. The photos, first analysed by the Institute for Science and International Security (ISIS), a US-based think tank, show tunnel entrances being strengthened at Mount Kolang Gaz La - also known as Pickaxe Mountain. Experts say this facility may be designed to protect Iran's uranium enrichment activities or key equipment, but its exact purpose - and whether it is operational - remains unclear. Iran insists its nuclear programme is entirely peaceful and denies it is seeking to develop nuclear weapons. However, Trump said Iran was as little as a month away from having a nuclear weapon in June 2025, when he ordered strikes on three underground nuclear sites where centrifuges were being used to produce highly enriched uranium during a 12-day war between Israel and Iran last year. Trump said the strikes had obliterated Iran's enrichment capacity, but that Iranian officials were thinking about starting a new site. Pickaxe Mountain was not one of the locations attacked in US and Israeli air strikes in June 2025, but the Natanz nuclear facility, located about 2km (1.24 miles) north, was hit. A satellite image from 10 February shows what appears to be fresh concrete laid on top of one of the Pickaxe Mountain entrance areas. A boom pump used to deliver concrete can be seen at the location, analysts from both ISIS and UK-based intelligence analysis firm Maiar agreed. At another tunnel entrance, rock and soil has been pushed back and flattened, with a new concrete-reinforced structure nearby. Experts at ISIS suggest these changes are intended to strengthen the tunnel entrances and give additional protection against a potential air strike. ISIS asserts that the ongoing presence of heavy construction machinery and material indicates the facility is not likely to be ready for operations yet. In the past, Iran has tied the construction to rebuilding an advanced centrifuge assembly plant, but the size of the facility, along with the protection provided by the tall mountain, raises immediate concern whether additional sensitive activities are planned, such as uranium enrichment. Recent satellite imagery also shows repair work and defensive strengthening has been taking place at the nearby Natanz nuclear site and at the Isfahan nuclear complex, 125km (77 miles) to the north. Expert analysis continues to suggest Iran is bolstering its defense mechanisms amid fears of further military strikes, keeping its nuclear ambitions on high alert.