More than 100 US military personnel are operating in Saudi Arabia, helping its military to select Houthi targets.

They are part of a joint command center established in recent weeks, to provide intelligence and real-time information, but they do not participate directly in strikes, CNN reported, citing five sources familiar with the operations. The U.S. forces also are not providing the kind of operational assistance they did in the past.

The US has helped Saudi Arabia use a targeting tool allowing military officers to see in real-time what is happening on the battlefield, one US official told CNN.

The joint command was established after U.S. intelligence indicated that there are currently hundreds of Iranian Revolutionary Guard Corps officers in Yemen, working alongside the Houthis with goal of ultimately shutting down the Bab-el-Mandeb strait, a key global chokepoint between Yemen on the Arabian Peninsula and Djibouti and Eritrea in the Horn of Africa.

The Houthis on Thursday seized Perim Island, a tiny but potentially crucial position at the narrowest point of the strategic Bab al-Mandab Strait. Houthi forces also have reached several other strategically important islands nearby, according to reports.

Bab al-Mandab links the Indian Ocean and Asia to the Red Sea, Suez Canal and Europe, making it a critical artery for global trade. Its importance has increased further since Iran closed the Strait of Hormuz after its war with the United States began in late February.

Saudi Arabia has responded with extensive airstrikes across Yemen in support of government forces. Houthi officials claimed Riyadh carried out around 40 strikes on Thursday morning alone in areas including Taiz, Hodeidah, Al-Jawf and Marib.

The escalation has effectively ended a four-year period of relative calm between Saudi Arabia and the Houthis.