WASHINGTON (Reuters) - The chairman of the U.S. Senate s East Asia subcommittee is asking China and 20 other nations on Monday to clamp down on North Korea by cutting ties, closing down diplomatic facilities and working to oust the country from the United Nations. As North Korea has defied U.N. Security resolutions to conduct nuclear and ballistic missile tests, Washington has been scrambling for an effective response. Many of President Donald Trump s fellow Republicans have been calling for tougher action, imposed more quickly, including calling for stiff  secondary  sanctions and other punishments for companies, banks and countries that do business with North Korea. In a letter seen by Reuters that will be sent to ambassadors from China and 20 other countries, Republican Senator Cory Gardner, the subcommittee chairman, asked that the countries immediately end diplomatic and economic relations with Pyongyang.  Maintaining official diplomatic relations with a regime that continues to defy international law and threaten nations across the globe only serves to reward nefarious behavior,  wrote Gardner.  In addition to cutting off bilateral ties, I urge your government to support expelling the DPRK from the United Nations,  the letter said. Gardner recently introduced legislation that would impose a U.S. economic embargo against any entity that does business with North Korea. The U.S. military staged bombing drills with South Korea over the Korean peninsula and Russia and China began naval exercises ahead of a U.N. General Assembly meeting on Tuesday where North Korea s nuclear threat is likely to loom large. Gardner s letter will be sent to ambassadors to the United States from China, Brazil, Britain, Bulgaria, Cambodia, Cuba, Czech Republic, Egypt, Germany, India, Indonesia, Laos, Malaysia, Mongolia, Nigeria, Pakistan, Poland, Romania, Russia, Sweden and Vietnam. 