A major expansion underway inside Iran’s most heavily protected nuclear facility could soon triple the site’s production of enriched uranium and give Tehran new options for quickly assembling a nuclear arsenal if it chooses to, according to confidential documents and analysis by weapons experts.

Inspectors with the International Atomic Energy Agency confirmed new construction activity inside the Fordow enrichment plant, just days after Tehran formally notified the nuclear watchdog of plans for a substantial upgrade at the underground facility built inside a mountain in north-central Iran.

Iran also disclosed plans for expanding production at its main enrichment plant near the city of Natanz. Both moves are certain to escalate tensions with Western governments and spur fears that Tehran is moving briskly toward becoming a threshold nuclear power, capable of making nuclear bombs rapidly if its leaders decide to do so.