Some Wall Streeters worry that Russia could use Friday's emergency United Nations Security Council meeting as a cover to launch an official invasion into eastern Ukraine, veteran trader Art Cashin told CNBC.
"Cynical traders got very nervous," said Cashin, UBS' director of floor operations at the New York Stock Exchange.
When Russia called for an emergency meeting of the U.N. Security Council on Friday, it ignited the collective cynicism of Wall Street and reversed what has been a good morning for stocks, Cashin said. Geopolitics was the "big surprise" that sent stocks lower, killing a modest rally caused by a strong April employment report, he said.
"That's cynical I know, but that's how traders think," Cashin said on "Squawk on the Street." "You saw gold spike. You saw the 10-year reverse sharply, the yield coming down."
Cashin said some traders believed Russia would ask for the U.N. to intervene in the escalating violence in eastern Ukraine, only to launch a major military operation when the U.N. declined.
Wall Street seemed so worried, Cashin said, that he told investors to watch whether 10-year Treasury yields make a major move to below 2.5 percent. That would signal a significant flight to safety off geopolitical concerns, Cashin said.
—By CNBC's Jeff Morganteen.