CHICAGO, May 11 — With primary election dates fast approaching in many states, officials in Pennsylvania and California issued urgent directives in recent days about a potential security risk in their Diebold Election Systems touch-screen voting machines, while other states with similar equipment hurried to assess the seriousness of the problem.

"It's the most severe security flaw ever discovered in a voting system," said Michael I. Shamos, a professor of computer science at Carnegie Mellon University who is an examiner of electronic voting systems for Pennsylvania, where the primary is to take place on Tuesday.

Officials from Diebold and from elections' offices in numerous states minimized the significance of the risk and emphasized that there were no signs that any touch-screen machines had been tampered with. But computer scientists said the problem might allow someone to tamper with a machine's software, some saying they preferred not to discuss the flaw at all for fear of offering a roadmap to a hacker.

"This is the barn door being wide open, while people were arguing over the lock on the front door," said Douglas W. Jones, a professor of computer science at the University of Iowa, a state where the primary is June 6.