As White House officials debate their approach for a meeting between President Trump and Russian President Vladimir Putin at the G20 summit in Germany next month, one veteran diplomat is speaking out about the "firm" message Washington needs to send to Moscow.

Former Ambassador William Burns CBS News

"This is not a time for chummy conversations," said former Ambassador William Burns, referring to the optics of Mr. Trump's meeting with Russia's foreign minister, Sergey Lavrov, in May. "There are a lot of very serious differences between us and Russia, and I think it's really important for the president to make that clear so there's no misunderstanding about the depth of our concern."

According to the Associated Press, Mr. Trump is "eager" to meet with Putin "with full diplomatic bells and whistles," but many in the Trump administration think it's best to maintain distance amid the ongoing investigation into Russia's meddling in the U.S. elections.

Burns, who served as the U.S. ambassador to Russia from 2005 to 2008 and retired from the State Department in 2014 as deputy secretary of state, shed light on Putin and why he aims to "chip away at American influence."

"I think Putin has a very combustible combination of grievance and insecurity and ambition. A lot of his worldview was shaped by the experience of Russia in the 1990s, the sense that Russia, which was then flat on its back economically, was being taken advantage of by the U.S. and the rest," Burns said. "And that the U.S. in that period reinforced an international order, from Putin's point of view, that constrained Russia, denied it what he thinks is the entitlement of a major power, which is his sphere of influence in the former Soviet space and gave license to the U.S. and other countries to question Putin's very tough authoritarian rule inside Russia itself. So as a result of that, Putin's conclusion is that the way to ensure Russia' major power status is chip away at American influence and also point to external threats – to say to Russians, it's the United States that's trying to keep us down and I'm the person who can stand up to them."

Burns said there was "a serious effort" by the U.S. to understand and help the Russians during the dissolution of the Soviet Union, but "the basic problem was a Russia that was going through a really difficult period, and there was a sense of humiliation that you couldn't escape."

Now the president of the Carnegie Endowment for International Peace, Burns said Mr. Trump needs to address the Russian hacking efforts first – "a very serious assault on our democratic system" – as well as continuing concerns over Russia's role in Ukraine and Syria.

On Monday night, the White House released a stern warning to Syrian President Bashar al-Assad claiming it had "potential" evidence that Syria was preparing another chemical weapons attack against its own people. Russia responded Tuesday, with the Kremlin spokesman telling reporters that Moscow considers "such threats against the Syrian leadership to be unacceptable."

According to Reuters, Assad also visited a Russian air base in western Syria on Tuesday.

"He's showing off. He's a thug," Burns said. "He's trying to demonstrate that he can get away with the subjugation of his people, having consolidated control in the western part of his country, most of it. He's determined to try to do it in the east as well."

There's "an awful lot at stake" in Syria, Burns said, adding, "it's a very combustible situation right now."