The South Korean army's K-55 self-propelled howitzers move during a military exercise in Paju, near the border with North Korea, Monday. Military tensions in the region continue to escalate.

SEOUL, South Korea — Following U.S. warnings to North Korea of a “massive military response,” South Korea fired missiles into the sea to simulate an attack on the North’s main nuclear test site on Monday, a day after North Korea detonated its largest-ever nuclear test explosion.

South Korea’s Defense Ministry also said Monday that North Korea appeared to be planning a future missile launch, possibly of an ICBM, to show off its claimed ability to target the United States with nuclear weapons, though it was unclear when this might happen.

The heated words from the United States and the military maneuvers in South Korea are becoming familiar responses to North Korea’s rapid, as-yet unchecked pursuit of a viable arsenal of nuclear-tipped missiles that can strike the United States. The most recent, and perhaps most dramatic, advance came Sunday in an underground test of what leader Kim Jong Un’s government claimed was a hydrogen bomb, the North’s sixth nuclear test since 2006.

The United Nations Security Council held its second emergency meeting about North Korea in a week on Monday, with U.S. Ambassador Nikki Haley saying the North’s actions show that Kim is “begging for war,” and the time has come for the Security Council to adopt the strongest diplomatic measures.

“Enough is enough. War is never something the United States wants. We don’t want it now. But our country’s patience is not unlimited,” Haley said.

In Seoul, Chang Kyung-soo, an official with South Korea’s Defense Ministry, told lawmakers on Monday that it was seeing preparations in the North for an ICBM test but didn’t provide details about how officials had reached that assessment.

Chang also said the yield from the latest nuclear detonation appeared to be about 50 kilotons, which would mark a “significant increase” from North Korea’s past nuclear tests.

Each new North Korean missile and nuclear test gives the country’s scientists invaluable information that allows big jumps in capability. North Korea is thought to have a growing arsenal of nuclear bombs and has spent decades trying to perfect a long-range missile to eventually carry smaller versions of those bombs.

Both diplomacy and severe sanctions have failed to check the North’s march to nuclear mastery.