DUBAI/ADEN (Reuters) - Saudi Arabia’s military in Yemen has moved in to secure two strategic Red Sea ports and the Bab al-Mandeb Strait after its leading ally the United Arab Emirates substantially reduced its presence there, four sources familiar with the matter said.

FILE PHOTO: A Yemeni soldier is pictured near a poster portraying Saudi Arabia's King Salman bin Abdulaziz Al Saud and Yemen's President Abdrabbuh Mansur Hadi outside a hospital renovated by Saudi Arabia in Aden, Yemen December 13, 2018. Picture taken December 13, 2018. REUTERS/Nael Shyoukhi/File Photo

The UAE has drawn down its numbers in some parts of Yemen, where it had set up large bases amid a four-year-old multi-layered war that is widely seen as a proxy battle between regional rivals Saudi Arabia and Iran.

Two Yemeni military commanders and two Yemeni government officials told Reuters that Saudi officers had taken command of military bases at the ports of al-Mokha and al-Khokha, which Emirati forces had used to back their campaign in nearby Hodeidah and to monitor the coastline.

Riyadh has also sent an unspecified number of troops to the southern port city of Aden and Perim Island, a small volcanic rock in the strategic Bab al-Mandeb shipping lane where the Red Sea meets the Gulf of Aden.

The spokesman for the Saudi-led coalition and the UAE government did not respond to requests for comment.

A senior Emirati official has said the UAE is not leaving a vacuum in Yemen as it trained about 90,000 local fighters and remains committed to the coalition and the Yemeni government. The UAE discussed extensively its troop redeployment with Riyadh, the official said.

The change in command is not expected to significantly affect the course of the war, nor a U.N.-backed ceasefire in Hodeidah which was signed last year in Sweden between the Saudi-backed government and the Iran-aligned Houthis.

Riyadh has remained supportive of U.N. peace efforts despite an uptick in Houthi missile and drone strikes on airports in southern Saudi Arabia over the last two months.

But by expanding the Saudi presence on the ground, the move could intensify international criticism of the kingdom’s role in a war that has unleashed the world’s worst humanitarian crisis and killed hundreds of civilians in errant air strikes on hospitals, schools and markets.

NARROW STRIP

The change coincides with growing tensions between Washington and Tehran, with the United States seeking to form a military coalition to protect international shipping from Iranian attacks off the coast of Yemen and near the Strait of Hormuz on the other side of the Arabian Peninsula.

On Thursday, three Iranian vessels tried to block a British tanker passing through Hormuz.

Iran has threatened to close the strategic waterway in response to U.S. calls to reduce its oil exports to zero.

Perim Island, in the middle of Bab al-Mandeb, was under the control of the UAE military since 2015, when the coalition intervened in Yemen’s war, until recent weeks as Saudi Arabia took over along with Yemeni coast guards, the sources said.

The UAE, a member of the Western-backed Sunni Muslim coalition battling the Houthis, and the Yemeni forces it supports had captured a series of western coastal towns to form a narrow strip of control.

Tens of thousands of Yemeni and Sudanese troops are positioned in the area and on the southern outskirts of Hodeidah, where warring parties agreed on a ceasefire to pave the way for talks to end the war.