South Korean officials detected signs recently that North Korea’s military was preparing for winter drills. The exercises would begin in December, according to South Korea’s Yonhap News Agency.

While months-long winter drills are routine for the North, authorities in the South said they were on high alert for any possible changes. South Korea was keeping its “full combat readiness against North Korea’s possibility provocations,” its military said Monday.

“Currently, the North Korean troops are conducting ordinary field exercises,” a South Korean military official said in a briefing, according to Yonhap. “But our military is keeping a full readiness posture against the possibility of North Korea’s provocations.”

South Korea and the United States were also conducting joint military drills in the East Sea utilizing three U.S. aircraft carriers. Those drills, set to end Tuesday, were aimed at sending a strategic warning to North Korea, officials said.

President Donald Trump, meanwhile, delivered a message to North Korean leader Kim Jong Un during a speech last week before the South Korean National Assembly.

“Do not underestimate us,” Trump said. “And do not try us. We will defend our common security, our shared prosperity and our sacred liberty. The regime has interpreted America’s past restraint as weakness. This would be a fatal miscalculation.”

The president also told Kim his nuclear proliferation was putting the regime in great danger, calling North Korea a “hell no person deserves.”

North Korea fired back, saying the U.S. was igniting nuclear war.

“Trump has flown to South Korea as he seeks to strengthen military threats against us and has an intention to light the fuse for a nuclear war,” North Korean newspaper Rodong Sinmum said. “The problem is that South Korea is blindly following the U.S. which is intent on the scheme for a nuclear war.”

As part of his continuing tour of Asia, Trump met with Japanese Prime Minister Shinzo Abe and Australian Prime Minister Malcolm Turnbull Monday in Manila to discuss the situation in North Korea. All three “reaffirmed their commitment to maintaining maximum pressure on North Korea in an effort to denuclearize the Korean Peninsula,” the White House said.