The US is sending American forces, including fighter aircraft, air defense missiles, and likely more than 500 troops, to a Saudi air base.

The Saudi Foreign Ministry announced the basing agreement Friday without mentioning details.

Senior US defense officials said some troops and Patriot air defense missile systems have already arrived at Prince Sultan Air Base, south of Riyadh, where the troops have been preparing for the arrival of aircraft later this summer as well as additional troops.

Putting US combat forces back in Saudi Arabia, after an absence of more than a decade, adds depth to the regional alignment of US military power, which is mostly in locations on the Persian Gulf that are more vulnerable to Iranian missile attack.

Visit Business Insider's homepage for more stories.

WASHINGTON (AP) — With Iranian military threats in mind, the United States is sending American forces, including fighter aircraft, air defense missiles and likely more than 500 troops, to a Saudi air base that became a hub of American air power in the Middle East in the 1990s but was abandoned by Washington after it toppled Iraqi strongman Saddam Hussein in 2003.

The Saudi Foreign Ministry announced the basing agreement Friday without mentioning details.

Senior American defense officials said some US troops and Patriot air defense missile systems have already arrived at Prince Sultan Air Base, south of Riyadh, where the troops have been preparing for the arrival of aircraft later this summer as well as additional troops. The officials spoke on condition of anonymity in order to provide details not publicly announced.

The agreement has been in the works for many weeks and is not a response specifically to Friday's seizure by Iran of a British tanker in the Persian Gulf. Tensions with Iran have spiked since May when the Trump administration said it had detected increased Iranian preparations for possible attacks on US forces and interests in the Gulf area.

Read more: 'Relax and speak to America': US soldiers in Trump's military parade are handed instructions on what not to say

In a written statement Friday evening, US Central Command said the deployments to Saudi Arabia had been approved by the Pentagon.

"This movement of forces provides an additional deterrent, and ensures our ability to defend our forces and interests in the region from emergent, credible threats," Central Command said. "This movement creates improvement of operational depth and logistical networks. US Central Command continually assesses force posture in the region and is working with Kingdom of Saudi Arabia authorities to base US assets at the appropriate locations."

Putting US combat forces back in Saudi Arabia, after an absence of more than a decade, adds depth to the regional alignment of US military power, which is mostly in locations on the Persian Gulf that are more vulnerable to Iranian missile attack.

But it also introduces a political and diplomatic complication for the Trump administration, accused by critics of coddling the Saudis even after the murder last fall of dissident writer Jamal Khashoggi by Saudi agents. Many in Congress now question the decades-old US-Saudi security alliance and oppose major new arms sales to the kingdom.

Starting with the January 1991 air war against Iraq after its invasion of Kuwait the previous summer, the US flew a wide range of aircraft from Prince Sultan air base, originally known as al-Kharj. Supported by an all-American array of creature comforts like fast-food restaurants and swimming pools, US forces there flew and maintained Air Force fighters and other warplanes.

The base also served as a launch pad for the December 1998 bombing of Iraq, code-named Operation Desert Fox, which targeted sites believed to be associated with Iraq's nuclear and missile programs. In 2001, the base became home to the US military's main air control organization, known as the Combined Air Operations Center, which orchestrated the air war in Afghanistan until it was relocated in 2003 to al-Udeid air base in Qatar.