A supercomputer developed by Riken, the state-backed research institute, has been surpassed in computing speed by a new U.S. supercomputer, according to a biannual ranking announced Monday by the U.S.-European TOP500 project.

The supercomputer, nicknamed "K," was developed last year by Fujitsu Ltd. and the Riken Advanced Institute for Computational Science in Kobe. It was knocked down to second place after topping the previous two rankings by the Sequoia, a supercomputer at the U.S. Department of Energy's Lawrence Livermore National Laboratory, the project said.

K was the first to log a computing speed of over 10 petaflops per second. Its name draws upon the Japanese word "kei," which means 10 to the 16th power and represents the system's performance goal of 10 petaflops. One petaflop is equivalent to 1,000 trillion operations per second.

The Sequoia, however, "achieved an impressive 16.32 petaflops/s on the Linpack benchmark," versus the K's 10.51 petaflops/s, the project said in a statement.

The entire system of the Riken-Fujitsu supercomputer, scheduled to be completed later this month by bundling 864 computers, is set to kick off full-fledged operations in late September.

K became a subject of controversy in Japan in 2009 after Democratic Party of Japan lawmaker Renho, seeking to cut state budget outlays on the project, questioned whether the country needed to pursue the No. 1 position in supercomputing.