The U.S. ambassador to South Korea called on North Korea to stop "provocative behavior" Thursday, saying the combined military capability of South Korea and the U.S. is strong enough to deal with any contingency on the peninsula. Ambassador Sung Kim made the call after the North fired 25 short-range missiles into waters off its east coast over the weekend, the latest in the show of force against the ongoing joint military drills between Seoul and Washington. The North has conducted several rounds of such military actions since Seoul and Washington began last month their annual military exercises, which run through April. The North has denounced the joint drills as a rehearsal for invasion, while the allies have said they are defensive in nature. "We have witnessed North Korea continue to improve its missile capability ... recently firing short-range missiles off its east coast," Kim said in a speech given to the American Chamber of Commerce in Korea. "We call on North Korea to cease provocative behavior."    Security challenges from North Korea are "real," the U.S. ambassador said, referring to North Korea's third nuclear weapons test last year, as well as recent short-range missile launches. "That's why we stand with Republic of Korea to deter, defend against any aggression," he said, referring to South Korea by its official name. He also reasserted U.S. military commitment to maintaining peace on the Korean Peninsula, saying, "You can be completely confident about our combined ability to deal with any contingency on the peninsula."    The ambassador also noted, "We will continue to coordinate closely with South Korea and our allies and partners to make North Korea make the right choices on denuclearization to build lasting peace on the peninsula."    Diplomatic efforts are now under way to revive the long-stalled six-party talks aimed at persuading Pyongyang to give up its nuclear weapons program, deemed one of most dangerous security threats in Asia. Seoul and Washington have said the multilateral forum also involving China, Russia and Japan could only resume when they are confident that a resumption will surely lead to denuclearization of North Korea.