It was a good thing for President Donald Trump to meet with other heads of state at the recent G-20 summit, but his focus on Russian President Vladimir Putin at a dinner party was "not a good form," former U.S. Ambassador to Russia Michael McFaul said Wednesday.

"You need to work the room," McFaul said during an interview on NBC's "Today" show. "You shouldn't just focus on one. How about a little time with the host, [German chancellor] Angela Merkel, our closest ally. Others were concerned about it."

Trump and Putin held a formal two-hour meeting on July 7, but the second, previously undisclosed conversation occurred during the dinner in Hamburg, said Ian Bremmer, the president of political risk consultancy Eurasia Group, who was first to report the meeting in a note to clients.

It was not disclosed what the two talked about, but they were accompanied only by a Russian translator, and that is an issue, the former ambassador said.

"As somebody that used to staff the meetings for President Obama when I worked at the White House, you want him to have his talking points," McFaul said. "If Putin says this, you say this. And anything a president utters in a meeting like that becomes policy. Those are the things you don't like to have happen without staff."

Putin, meanwhile, is "not a big chitchat guy," McFaul pointed out.

"He is going to take advantage of the situation to push his agenda, which is concessions from the United States, [on things like] sanctions," McFaul said, "and he has 17 years of experience in doing foreign policy. Our president has a few months."