Art Cashin said investors should be wary of increased volatility related to worries about Russia and Ukraine near the close of Friday's stock trading
Cashin, UBS' director of floor operations at the NYSE, told CNBC's Bob Pisani around midday that "the rumormongers may come out late in the session" and spark sharp moves in the market.
"This is a perfect atmosphere for it. You're going into the weekend, Europe is basically closed. It's tough to check details out so it's easy to get things (rumors) out there."
Cashin said if there is a move downward, he'll be watching Thursday's low of 1841 on the S&P 500 and "important support" at 1834.
—By CNBC's Alex Crippen. Follow him on Twitter