The Pentagon announced on Wednesday that the navy’s top civilian official, John Phelan, the secretary of the navy, is leaving his job.

In a statement posted to social media, Sean Parnell, a Pentagon spokesperson, said Phelan was “departing the administration, effective immediately”.

Hung Cao, the navy undersecretary, will become acting secretary of the navy, Parnell said.

People familiar with the dynamics at the Pentagon told the Guardian Phelan was fired. Phelan had an increasingly rocky relationship with defense secretary Pete Hegseth and other senior staff, who openly appeared to prefer Cao for the role.

The sources said Hegseth in effect blamed Phelan for not going aggressively enough against senator Mark Kelly after appearing in a video in which the senator advised troops to ignore “illegal orders”.

Phelan also reportedly clashed with Hegseth’s deputy, Stephen Feinberg, whom the New York Times and CNN reported disapproved of Phelan’s handling of shipbuilding initiatives.

The sudden departure comes just a day after Phelan addressed a large crowd of sailors and industry professionals at the navy’s annual conference in Washington DC, and spoke with reporters about his agenda.

Phelan’s departure also comes just weeks Pete Hegseth fired the army’s top officer, Gen Randy George. Hegseth also has fired several top generals, admirals and other defense leaders since taking office last year. As with many of those other firings, Pentagon officials did not offer a reason for Phelan’s departure.

Phelan is leaving just as the US navy has imposed a blockade of Iranian ports and is targeting ships linked to Tehran around the world during a tenuous ceasefire in the Iran war.

Phelan had not served in the military nor did he have a civilian leadership role in the service before Donald Trump nominated him for secretary in late 2024.

Phelan was a major donor to Trump’s campaign and founded the private investment firm Rugger Management LLC. According to his biography, Phelan’s primary exposure to the military came from an advisory position he held on the Spirit of America, a non-profit that supported the defense of Ukraine and Taiwan.

The Associated Press could not immediately reach Phelan’s office for comment.

Cao ran a failed US Senate bid in Virginia to try to unseat Tim Kaine, a Democratic senator, Tim Kaine in 2024. He had Trump’s endorsement in the crowded Republican primary.

Cao is a 25-year navy veteran who served in combat zones. His biography includes fleeing Vietnam with his family as a child in the 1970s. In a campaign video for his Senate bid, he compared Vietnam’s communist regime during the cold war to the administration of former president Joe Biden.