US President George W. Bush, seeking a united stand against North Korea, called the leaders of China and Russia on Thursday as he stepped up pressure on communist regime in Pyongyang to stop its missile tests and nuclear weapons programs. The calls to Chinese President Hu Jintao and Russian President Vladimir Putin followed phone conversations Wednesday evening with the leaders of South Korea and Japan, the White House said. The administration brushed off blustering warnings from North Korea. "We're certainly not going to overreact ... to these wild statements out of Pyongyang and North Korea," said Undersecretary of State R. Nicholas Burns. "We've seen them before."