NEC Corp. has announced the world's fastest supercomputer, a machine that can carry out more than 20 billion operations per second and spells trouble for the American lead in supercomputers.

NEC's SX-X series will be so much faster than American-made machines that U.S. commercial and government customers may feel they have no choice but to buy the Japanese machine, said George Lindamood, an analyst at the Gartner Group consulting firm in Stamford, Conn."I imagine there will be some fairly severe government wrangling in Washington as a result of this one," Lindamood said.

The SX-X series will go on sale in the July-September period of 1990, NEC said. That is about the same time Cray Research Inc., the Minneapolis-based world leader in supercomputers, begins delivery of its Cray 3 models.

NEC said the SX-X series can attain peak vector speeds of 20 billion floating-point operations per second. Floating-point operations are high-precision calculations on rows and columns of numbers.

In contrast, the Cray 3 is expected to perform up to 16 billion operations per second, Lindamood said.

The computers are being sold in North America by HNSX Supercomputers Inc., a marketing company based in Burlington, Mass., and jointly owned by NEC and Honeywell Inc. They will range in price from $5 million to $23 million and will use a version of the Unix base layer of software.