Watch Art Cashin: Why stocks retreated from highs

Art Cashin said "risk is back in" the market, taking stocks off early highs sparked by weaker-than-expected U.S. financial sanctions against some Russian government officials and companies.

Cashin, UBS' director of floor operations at the NYSE, told CNBC's Bob Pisani around midday the market is "nervous" amid reports of scattered violence in Ukraine.

Cashin noted that the Nasdaq, an index he called the "weak link," had dropped into negative territory. Traders are looking at 4060 as a technical support level, he said. "They think If it breaks below 4060, it can accelerate on the downside, and that may pull some of the other markets lower."

—By CNBC's Alex Crippen. Follow him on Twitter @alexcrippen.