A group of more than 1,100 U.S. military veterans from all five branches have signed a statement lashing out at President Donald Trump for firing Lt. Col. Alexander Vindman from the National Security Council.

Vindman testified before a House committee during the Trump impeachment hearings in November. He expressed his concerns about Trump's drive to push Ukraine to investigate Democratic presidential candidate Joe Biden.

The Military Times newspaper published the statement, whose signatories invited other veterans to add their names.

The statement said the president's "actions and insults" toward Vindman "appear to be motivated by nothing more than political retribution, and deprives the White House of expertise necessary to defend our collective national security."

Military code of conduct

Long-standing U.S. military code of conduct requires servicemen and women to report wrongdoing and illegal acts through the proper military channels. But U.S. law forbids them from speaking out in public.

The veterans say Trump knows this and believes he can verbally attack Vindman with "impunity."

"We consider President Trump's sustained attacks on an active duty Army officer ... to be an affront to the constitution that we have all sworn to uphold. We are speaking out precisely because neither LTC Vindman nor his fellow active-duty service members can," the statement said.

The veterans' statement also criticizes what they say is Trump's association with those they call war criminals, his public threat of war crimes, and minimizing the traumatic brain injuries some troops suffered in January's Iranian missile attack on a military base in Iraq.

White House response

The White House has not yet responded to the statement.

The Ukrainian-born Vindman was the NSC's Director for European Affairs until he was reassigned three weeks ago.

The White House said Vindman was not fired and gave the official reason for his reassignment as downsizing within the NSC.

However, Trump has publicly accused Vindman of being a poor worker who did "a lot of bad things," including allegations of leaking classified information -- charges Vindman's supporters deny.