WASHINGTON, Oct 31 (Reuters) - North Korea on Monday demanded that the United States and South Korea stop large-scale military exercises, calling them a provocation that may draw "more powerful follow-up measures" from Pyongyang.The United States and South Korea began one of their largest combined military air drills on Monday, with hundreds of warplanes from both sides staging mock attacks 24 hours a day for the better part of a week.On Friday, South Korean troops finished the 12-day Hoguk 22 field exercises, which featured mock amphibious landings and river crossings, including some drills with U.S. forces.North Korea condemns the joint drills as a rehearsal for invasion and proof of hostile policies by Washington and Seoul.He was asked at a news briefing about comments last week by a senior U.S. official responsible for nuclear policy who raised some eyebrows by saying Washington would be willing to engage in arms-control talks with North Korea, something some experts say would require recognizing North Korea as a nuclear-armed state."