The United States said Monday that the firing of more than 100 artillery shells by the Democratic People's Republic of Korea (DPRK) is not a "helpful sign," threatening to continue mounting pressure on Pyongyang to change course.

"It is not a helpful sign by North Korea and this is exactly the kind of behavior that we would like to see North Korea avoid," State Department spokesman Philip Crowley told reporters at the daily press briefing.

"What we can continue to communicate to North Korea is that there will be no reward for these provocations. North Korea will continue to be isolated. We will continue to work with the international community to fully implement the Resolution 1874," said Crowley.

The DPRK on Monday fired more than 100 artillery shells into waters off the west coast of the Korean Peninsula, the South Korean Ministry of National Defense confirmed to Xinhua.

Given that all the shells dropped into waters north of the Northern Limit Line (NLL), and did not cause any damage to the South side, the South Korean military only sent warning messages without firing warning shots in response to the DPRK's shootings, and has been keeping an eye on further moves by the DPRK, the South Korean defense ministry said.

The artillery firing came as South Korea's five-day military drill in the Yellow Sea, which took the DPRK as the simulated enemy, entered the final day.

"We want to see North Korea cease its provocative actions. We want to see North Korea find a way to work constructively with its neighbors. We want to see North Korea reduce tensions," said Crowley.