Donald Trump's disgraced ex-National Security Advisor Mike Flynn is showing 'all the signs' of being an FBI co-operating witness, a senator claimed Monday.

Rhode Island Democrat Sheldon Whitehouse told CNN that there was every reason to believe the retired Army general was now helping the feds in the probe into claims of collusion between the Trump campaign and Russia.

Flynn was fired in disgraced just a month into Trump's presidency for misleading Vice President Mike Pence about his conversations with Russia's ambassador to the U.S., Sergey Kislyak.

Whitehouse said that it appeared likely that Flynn's apparent untruths to the FBI in the course of the investigation were enough to guarantee him a felony conviction, allowing the feds to 'flip' him.

He told CNN's Wolf Blitzer: 'All the signals are suggesting he [Flynn] is already cooperating with the FBI, and may have been for some time.

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Dramatic suggestion: 'All the signals are suggesting he [Flynn] is already cooperating with the FBI, and may have been for some time,' Sheldon Whitehouse told Wolf Blitzer

In the crosshairs: Flynn could be an FBI co-operating witness, a member of the Senate suggested - while two powerful House Democrats demanded answers about his part in a bizarre plan to get Arab countries to buy $100 billion in Russian arms

'First of all, they had him dead to rights on a felony false statement, on the statement they took from him at the White House on the Kislyak conversations.'

Whitehouse said that Flynn had also submitted new documents detailing his activities on the behalf of foreign governments, which he claimed was something which the FBI asks cooperating witnesses to do.

And he claimed that their were a series of subpoenas issued already, in which Flynn was not named making him 'the hole in a donut of subpoenas'.

'It could be a huge deal. Who knows what Trump has said to him?' the senator said.

'And apparently, Trump has been in touch with him after his firing from the White House to tell him to 'Stay strong.' Which in some circumstances could be looked at as manipulation of a witness, or obstruction of justice.'

The extraordinary claims from the senator - which cannot be verified - came as details were revealed of how Flynn may have been involved in a bizarre plan to get Arab countries to buy Russian arms in return for gaining nuclear technology.

The two most senior Democratic members of the House Oversight Committee and Foreign Affairs Committees have written to Flynn to ask for more details of trips he took to Saudi Arabia, Egypt and Israel in 2015 and which was not disclosed in his initial official declaration of foreign involvement.

Elijah Cummings, from oversight, and Eliot Engel, from foreign affairs, have demanded a full set of documents on the plan, ABC News reported.

The plan, first reported earlier this month in Newsweek, involved persuading Saudi Arabia to fund a $1 trillion network of nuclear power plants in Arab countries.

The nuclear technology would be from the United States and Russia. In return, the countries buying the technology would also buy $100 billion in Russian arms.

Letter: Elijah Cummings (center) is one of the two House Democratic ranking committee members demanding answers from Flynn

Reason for firing: Flynn was get rid of by President Trump over his lies about conversations with Sergey Kislyak, the Russian ambassador to the U.S. who is seen as a top intelligence asset for Vladimir Putin's Kremlin - but who was welcomed into the Oval Office in May

Russia had been planning to sell the arms to Iran, but the scheme's idea was that Moscow would be willing to end its involvement with Tehran in return for the nuclear and arms deal.

The company promoting the scheme, X-Co Dynamics Inc./Iron Bridge Group, run by a group of retired generals and equivalent ranks from other services, had Flynn involved to advise on producing a security plan.

But the letter from Cummings and Engel details a series of problems with his declaration which they want to see documents for.

The letter suggests the first summer 2015 trip, which took him to Israel and Egypt, was not disclosed at all.

And the second, later that year, to Saudi Arabia was declared but with missing or unverifiable details.

Among the problems were that the 'friend' he traveled with was not named, the speaking engagement he claimed could not be confirmed with the agencies which usually booked his appearances, and he claimed to stay at a hotel - the King Khaled International Hotel - whose existence could not be proven by investigators working for Congress.

Flynn’s attorney told ABC News he would not comment on the new allegations.

The two reps wrote: 'Most troubling of all, we have no record of Gen. Flynn identifying on his security clearance renewal application – or during his interview with security clearance investigators – even a single foreign government he had contact with.

'Gen. Flynn failed to disclose these contacts with Saudi or other foreign officials on his security clearance application or during his interview with security clearance investigators which could constitute further violations.'

Flynn has already been under fire for failing to initially disclose payments from the Turkish government, and how he was paid for appearing in Moscow when he had dinner with Vladimir Putin.