After less than three months on the job, acting US State Department Inspector General Stephen Akard, whose predecessor was abruptly fired by President Donald Trump in May, stepped down from the post Wednesday to return to the private sector, said the agency.

Akard "left to go back home", US Secretary of State Mike Pompeo said at a news conference Wednesday. "This happens. I don't have anything more to add to that."

The Office of the Inspector General of the Department of State is the agency's internal oversight watchdog, which has hundreds of employees and is responsible for investigating fraud, waste, abuse and mismanagement within the State Department.

Announcing the departure, a State Department spokesman said Akard's deputy, Diana Shaw, would become the new acting inspector general.

Akard, an ally of Vice-President Mike Pence, once headed the Indiana Economic Development Corp when Pence was governor of that state.

Akard also previously was named head of the State Department's Office of Foreign Missions in September, an ambassador-level title he retained over the past three months, even though that position comes under the oversight of the inspector general, a potential conflict of interest that drew criticism from congressional Democrats and others.

Akard's departure comes after his predecessor, Steve Linick, who had joined the State Department in 2013, was ousted May 15, the latest in a series of government watchdogs dismissed by Trump, decisions that have raised alarm among Democrats and some Republicans.

Congressional Democrats have launched an investigation into Trump's firing of Linick, which he said he had done at the behest of Pompeo, one of his most trusted cabinet members.

Democratic lawmakers have said Trump might have fired Linick because he was investigating $8.1 billion in US military sales to Saudi Arabia and the United Arab Emirates.

Congressional aides have also said Linick was probing whether Pompeo had misused a taxpayer-funded political appointee to perform personal tasks for himself and his wife, such as walking their dog. Pompeo dismissed the accusations as "crazy stuff" and denied knowing the scope of Linick's probe.

Senator Bob Menendez, the lead Democrat on the Foreign Relations Committee, said that Akard's sudden resignation left another opportunity for the administration to try to weaken oversight, even if he did not consider Akard the right choice for the inspector general role.

"Independent, experienced inspectors general are paramount to effective oversight," Menendez said in a statement. "I do not believe he was the right choice to lead the office, but I am concerned that his sudden resignation leaves another opportunity for the Trump administration to try to weaken oversight and accountability."

Menendez also said investigation into Linick's firing would continue. On Monday, senior congressional Democrats announced subpoenas for depositions from four aides to Pompeo, two of whom, Brian Bulatao and Toni Porter, are longtime close friends of Pompeo and his wife, Susan, according to a report in The Washington Post.

Lawmakers and aides intend to ask them about their actions and those of Pompeo in pushing through the Mideast arms deal, as well as Linick's inquiry into the issue.

Reuters contributed to this story.