U.S. Calls for End to North Korea's 'Provocative Actions' Against South

U.S. Defense Secretary Chuck Hagel condemned North Korea’s decision Monday to fire rockets into South Korean waters as “provocative actions” that must stop.

During a maritime military exercise, North Korea fired some 100 artillery shells across a boundary off the peninsula’s western coast. South Korea responded with more than 300 rounds of artillery fire into waters north of the boundary.

A South Korean official stressed they were not shooting at North Korea, but into open waters.

This exchange of fire comes on the heels of North Korea’s warnings it may resume conducting nuclear weapons tests.

Senior defense and intelligence officials tell NBC News they have seen no indications the North Koreans are preparing for any nuclear testing.

As for the live-fire exchange, defense officials report U.S. military in the region are closely monitoring the situation and have seen no signs of escalation between North and South.

The State Department also expressed their "strong concern" over North Korea's actions Monday.

State Department spokesperson Marie Harf called on the North "to cease and desist from needlessly threatening regional peace and security."

Harf presented Pyongyang with a choice: "they can choose to further escalate, or they can choose to come in line with their international obligations and rejoin the international community."

— Jim Miklaszewski, Courtney Kube and Catherine Chomiak