(NewsNation Now) — With an estimated 190,000 Russian troops amassed along the border with Ukraine, U.S. officials have been signaling a Russian invasion could begin any moment.

It is unclear, however, how the beginning of an invasion will be defined or what it will look like as some say a Russian incursion on Ukraine’s sovereignty has been ongoing for some time.

“Since 2014 … there’s been a low-scale war going on,” NewsNation’s Leland Vittert said during an appearance on “Morning in America”.

According to Vittert, what an invasion might look like will be a surprise “for anyone who thinks that we’re going to see the pictures of D-Day … of tanks rolling across the Ukrainian border.”

Vittert doesn’t think “somebody is going to get on television and say, the invasion has begun.”

Vittert spent time reporting from within Ukraine in 2014, when Russia-backed militias were trying to take over parts of the country.

Crimea is a peninsula along the northern coast of the Black Sea in Eastern Europe, and in 2014 Russia invaded and subsequently annexed the Crimean Peninsula from Ukraine.

In 2014, Russian President Vladimir Putin sent in the same kind of “expeditionary forces that we’re going to see in — probably already are seen in — eastern Ukraine, into Crimea, and in what’s called the Donbas,” Vittert said.

The Donbas region “is where sort of ethnic Russians traditionally live in Ukraine,” Vittert said, as some of the territory is occupied by separatist states.

Heavy shelling continued Monday in this eastern Ukrainian region as worries grow Putin will use the skirmishes as a pretext for an all-out attack.

Putin went as far Monday to convene top officials to consider recognizing the independence of these Russia-backed separatist regions.

Recent video from inside eastern Ukraine showed there are still daily attacks on Ukrainian military positions.

“The Russians continue to probe,” Vittert says. “And the Russian separatists, which is this group of militia inside eastern Ukraine, backed by the what was the KGB — now it’s called the FSB or the GRU — are already laying the groundwork for the formal Russian military to come in.”

According to Vittert, “It’s not in anybody’s interest to declare the invasion is started.”

“If the United States declares the invasion is starting, they admit failure,” he said. “If the Russians say the invasion is started, well, then they admit that they’re invading a sovereign country for no reason effectively other than Vladimir Putin wants to.”

And if the Ukrainians declare the start of an invasion, Vittert says, “all of a sudden, their economy gets even worse than it is and the president’s even in a more untenable position than he is.”

According to Vittert, it’s in everyone’s best interest to keep up what he calls this charade that somehow diplomacy is still an option.

The White House has shown that it values its diplomatic efforts, and Putin has verbally expressed interest in that, too.

“Because as long as he dangles this carrot to the United States, that look, ‘I haven’t invaded, even though I’m shelling and bombing Ukrainian cities.’ So long as you say, ‘I haven’t invaded and I say I haven’t invaded, then we can all talk’ and Vladimir Putin knows that the United States and the American public place this huge value on the idea that diplomacy is still alive.”

Watch the full interview with Leland Vittert in the video player at the top of the page.