WASHINGTON, May 3 (Reuters) -

The United States acted on Friday to force Iran to stop producing low-enriched uranium and expanding its only nuclear power plant, intensifying its campaign aimed at halting Tehran's ballistic missile program and curbing its regional influence.

At the same time, the Trump administration renewed waivers of U.S. sanctions allowing Russia, China, France and Britain to pursue programs designed to prevent Iran from reactivating a defunct nuclear weapons program, the State Department said in a statement.

Instead of granting the waivers for 180 days, the administration will shorten their term to 90 days, the statement said.

Under the 2015 nuclear deal with six major powers, Iran agreed to transform its nuclear facilities at Arak and Fordow in ways that would make it much more difficult to use them to produce fissile material for nuclear weapons.

The four nations' ability to continue that work, however, was compromised by U.S. President Donald Trump's decision last year to abandon the agreement, which was designed to limit Iran's nuclear program in return for economic sanctions relief.