Investigations have intensified into President Donald Trump's sacked national security adviser, Michael Flynn, with the Pentagon watchdog joining lawmakers in probing payments he accepted from foreign sources including a Russian state-sponsored TV network.

At the same time, documents released on Thursday by the top Democrat on a House oversight committee showed Flynn was warned by authorities after he retired from the military in 2014 not to take foreign government-sourced money without "advance approval" from the Pentagon.

Flynn, a former army lieutenant general and Defense Intelligence Agency chief, later accepted tens of thousands of dollars for his work on behalf of foreign interests, including RT, the state-supported Russian television network, and a Turkish-owned company linked to Turkey's government.

The Pentagon's acting inspector general's office confirmed on Thursday he has launched an inquiry into whether those payments qualify as coming from foreign governments and whether Flynn properly informed military authorities about them.

Peter Mathews, a political science professor at Cypress College, said Flynn should have exercised extra caution as former officers such as him can be called back to serve active duty at any time, for example if war breaks out.

"During the time they are retired, they shouldn't have any obligations to foreign governments or countries by taking money from those foreign sources," Mathews told Al Jazeera.

Trump fired Flynn in February for failing to inform senior administration officials about his contacts with Russian officials - contacts that are being examined as part of the wider inquiries into Russian meddling in the 2016 presidential campaign.

The White House defended its hiring of Flynn and attempted to shift blame for any problems with his vetting onto the Obama administration, which handled the reissuance of his security clearance in January 2016.

'Bold warning'

Elijah Cummings, who released the documents showing Flynn was warned, said during a news conference that Flynn had been clearly informed he needed to get permission to receive foreign payments and there is no evidence he did so.

"The Pentagon's warning to General Flynn was bold, italicised and could not have been clearer," Cummings said.

In a key 2014 document, Flynn was told by a Defense Intelligence Agency official that the US constitution's emoluments provision prohibits any monetary payments or gifts "from a foreign government unless congressional consent is first obtained".

Earlier this week, Cummings and Republican Jason Chaffetz, chairman of the House committee, said that they had found no evidence Flynn asked the army for permission to receive foreign payments or informed the military he had accepted them.

Army spokeswoman Cynthia O. Smith said the Army had no records that Flynn requested that permission.

One episode in question involves a trip he took to Moscow in 2015 for RT's anniversary celebration. He was paid at least $33,750 to attend the gala at which he was seated next to Russian President Vladimir Putin.

Turkish payments

In a statement, Flynn's attorney Robert Kelner said the defense department "was fully aware of the trip," citing his previous statements that his client briefed DIA officials before and after the RT trip.

Kelner also pointed to a letter, released in redacted form by Cummings, that notes that Flynn provided a thumb drive to the agency containing documents detailing the RT event, including that Leading Authorities, a speakers bureau that handled Flynn's paid speeches, was handling the event for him.

Flynn has previously disclosed he got between $50,000 and $100,000 as part of his personal stake in $530,000 that his company, Flynn Intel Group, received for consulting work last year for a Turkish businessman.

Last month Flynn's firm filed as a foreign agent with the justice department for its consulting work and acknowledged the work may have benefited the government of Turkey.

Flynn's client, Inovo BV, is owned by a businessman who is also a member of a committee overseen by Turkey's finance ministry.

In comments to the Associated Press, Utah Republican Chaffetz, chairman of the Committee on Oversight and Government Reform, said that Flynn "had an obligation to seek approval to take money from a foreign government. We found no evidence that he did that."

The committee's inquiry is one of several congressional investigations into Flynn's contacts with foreign officials.

On Thursday, White House press secretary Sean Spicer rebuffed criticism from Cummings that the White House was covering up. Asked about Trump administration vetting, Spicer appeared to shift blame onto the Obama administration, which had fired Flynn as head of the DIA.

"Why would you re-run a background check on someone who is the head of the Department of Defense Intelligence Agency that had and did maintain a high-level security clearance?" Spicer asked.

He noted that Flynn's security clearance was renewed during the Obama administration "with all of the information that's being discussed that occurred in 2015".