The United States is expected to designate Iran's elite Revolutionary Guards Corps a foreign terrorist organization, three US officials told Reuters, marking the first time Washington has formally labeled another country's military a terrorist group.

The decision, which critics warn could open US military and intelligence officials to similar actions by unfriendly governments abroad, is expected to be announced by the US State Department, perhaps as early as Monday, the officials said. It has been rumored for years.

The Pentagon declined comment and referred queries to the State Department. The State Department and White House also declined to comment.

The Iranian mission to the United Nations did not immediately respond to a request for comment.

US Secretary of State Mike Pompeo, a strident Iran hawk, has advocated for the change in US policy as part of the Trump administration's tough posture toward Tehran.

The announcement would come ahead of the first anniversary of President Donald Trump's decision to pull out of a 2015 nuclear deal with Tehran and to reimpose sanctions that had crippled Iran's economy.

The administration's decision to make the designation was first reported by the Wall Street Journal.

The United States has already blacklisted dozens of entities and people for affiliations with the IRGC, but the organization as a whole is not.

In 2007, the US Treasury designated the IRGC's Quds Force, its unit in charge of operations abroad, "for its support of terrorism," and has described it as Iran's "primary arm for executing its policy of supporting terrorist and insurgent groups."

Iran has warned of a "crushing" response should the United States go ahead with the designation.

IRGC commander Mohammad Ali Jafari warned in 2017 that if Trump went ahead with the move "then the Revolutionary Guards will consider the American army to be like Islamic State all around the world."

Such threats are particularly ominous for US forces in places such as Iraq, where Iran-aligned Shi'ite militia are located in close proximity to US troops.

Republican Senator Ben Sasse said the move would be an important step in America's maximum pressure campaign against Tehran. "A formal designation and its consequences may be new, but these IRGC butchers have been terrorists for a long time," Sasse said in a statement.

Former Under-Secretary of State and lead Iran negotiator, Wendy Sherman, said she worried about implications for US forces.

"One might even suggest, since it's hard to see why this is in our interest, if the president isn't looking for a basis for a conflict," said Sherman, who is director of the Center for Public Leadership at the Harvard Kennedy School. "The IRGC is already fully sanctioned and this escalation absolutely endangers our troops in the region."

IRGC'S REACH

Set up after the 1979 Islamic Revolution to protect the Shi'ite clerical ruling system, the IRGC is Iran's most powerful security organization. It has control over large sectors of the Iranian economy and has a huge influence in its political system.

The IRGC is in charge of Iran's ballistic missiles and nuclear programs. Tehran has warned that it has missiles with a range of up to 2,000 km (1,242 miles), putting Israel and US military bases in the region within reach.

The IRGC has an estimated 125,000-strong military with army, navy and air units and answers to Supreme Leader Ayatollah Ali Khamenei.

It is unclear what impact the US designation of the IRGC as a terrorist organization might have on America's activities in countries that have ties with Tehran, including in Iraq.

Baghdad has deep cultural and economic ties with Iran and Oman, where the United States recently clinched a strategic ports deal.