Issuing fresh nuclear weapons warnings, North Korea over the weekend threatened the United States with the "strongest military counter-action" should a major South Korea-U.S. joint military exercise go forward.Monday saw the start of the two week-long "Ulchi Freedom" drill which involves tens of thousands of South Korean and U.S. troops in a war game that simulates an invasion by North Korea, reported news service AFP.The North Korean army and people "are no longer what they used to be in the past when they had to counter the U.S. nukes with rifles," the powerful National Defense Commission said in a statement.It is now an "invincible power equipped with both latest offensive and defensive means ... including nuclear deterrence," it said. 4951