Sen. Tim Kaine is calling for the release of a secret document detailing President's Donald Trump's interpretation of his powers to go to war, NBC News is reporting.

Kaine, D-Va., has been pushing for Trump to outline his justification for bombing Syria last April following President Bashar al-Assad's chemical attacks on civilians. Increased U.S. involvement in Syria and heightened tensions with North Korea have added to Kaine's urgency to obtain the memo, NBC News reported.

The existence of the memo came to light last fall after a Freedom of Information Act filed by Protect Democracy sought the president's legal justification for the strikes, NBC News said.

In a letter to Secretary of State Rex Tillerson, Kaine, who sits on the Armed Services and Foreign Relations committees, requested the 7-page memo the administration has closely guarded for months.

"The fact that there is a lengthy memo with a more detailed legal justification that has not been shared with Congress, or the American public, is unacceptable," Kaine said in the letter.

"I am also concerned that this legal justification may now become precedent for additional executive unilateral military action, including this week's U.S. airstrikes in Syria against pro-Assad forces or even an extremely risky ‘bloody nose' strike against North Korea."

Kaine's letter follows a letter sent to Trump by Sen. Martin Heinrich, D-NM, saying the president lacks the "legal authority" to carry out a preemptive strike against North Korea, the network news said.

"Unless Donald Trump goes to Congress before starting a new war, the real bloody nose is going to be the American Constitution," said Allison Murphy, counsel at Protect Democracy.

Murphy had served in President Barack Obama's White House Counsel's office.

"Congress needs to demand the secret Syria memo when the administration is threatening to use force around the world without authority. The American people deserve to see" it, Murphy said.

Kaine has said the president can defend the U.S. against an imminent attack, but the nation has never used nuclear weapons in a first strike.

"He fired missiles at Syria in April without coming to Congress and still hasn't given us any legal rationale for that missile strike," he told MSNBC's "Andrea Mitchell Reports." "The thought that he could think his nuclear button is big and he has the power to do something without Congress should trouble everybody."