Aranyaprathet, Thailand — A bombing run by two Thai jets forced Vietnamese troops to retreat to Kampuchea from positions inside Thailand, military officials said. Intelligence sources here said Vietnam's offensive, which entered its sixth day Tuesday, apparently stalled following the Thai air strike. But Thailand was taking no chances. It ordered fresh troops to the border, military sources said, and all forces had been put on full alert. The United States has expressed concern about increasing violence on the ill-defined border.