Nicholas Burns, formerly U.S. ambassador to NATO, took President Donald Trump to task on Wednesady for taunting Russia about a possible missile strike on Syria.

"This is not a video game. The president of the United States needs to be calm and needs to act presidential," said Burns, who has advised Republican and Democratic presidents and is now a professor at Harvard's Kennedy School of Government. "The tweet this morning was irresponsible. And it wasn't presidential. It wasn't effective."

Hours earlier, Trump took to Twitter to strongly suggest a missile strike on Syria in response to its alleged chemical weapons attack on a rebel-held town.

Russia vows to shoot down any and all missiles fired at Syria. Get ready Russia, because they will be coming, nice and new and "smart!" You shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it!

"There's never been an American president going back to the dawn of the nuclear age that was goaded ... like this," Burns said on CNBC's "Squawk Box."

In a second tweet, Trump wrote the U.S. relationship with Russia "is worse now than it has ever been."

Our relationship with Russia is worse now than it has ever been, and that includes the Cold War. There is no reason for this. Russia needs us to help with their economy, something that would be very easy to do, and we need all nations to work together. Stop the arms race?

Burns, who has recently been giving Trump credit for diplomatic efforts with China and North Korea, said the White House needs to separate any U.S. response in Syria and the American relationship with Russia, which has been militarily supporting the government of Syrian President Bashar Assad.

"There's no question that the Russians are wrong to support the Syrian government," said Burns. "They are a real problems for us. But we're talking about airstrikes here."

"I will support the use of airstrikes against Syria," Burns said. "But you don't want to mix Russia into this. You don't want the Russians to think the United States is targeting them."

"I would separate the two issues. And I believe the president probably is," Burns said. "[But] the president did not do that well this morning."

Burns was U.S. ambassador to NATO and the State Department's third-ranking official during George W. Bush's presidency. He also advised the administrations of George H.W. Bush and Bill Clinton.