Seoul, South Korea ? North Korea fired about 110 rounds of artillery Monday near its disputed sea border with South Korea, the South’s military said, amid high tension over the deadly sinking of a South Korean warship blamed on North Korea.

The firing came shortly after South Korea ended naval drills off the west coast that the North called a rehearsal for an invasion, vowing to retaliate.

All the artillery shells harmlessly landed into the North’s waters and caused no damage to the South, a South Korean Joint Chief of Staff officer said on condition of anonymity, citing department policy.

South Korea considered the firing to be part of a military drill by North Korea but still bolstered its military readiness against further provocation, the officer said. The South also warned Pyongyang over the firing by naval radio, he said.

“This was their way of saying ‘We’ll respond to military drills with military drills,”‘ said Yang Moo-jin, a professor at Seoul’s University of North Korean studies.

Yang said the firing is also aimed at highlighting the instability of the Korean peninsula to apply pressure on the United States to start talks on the signing of a peace treaty to formally end the 1950-53 Korean War. The conflict ended with an armistice, thus leaving the peninsula technically at state of war.

A U.S. State Department spokesman denounced North Korea’s firing of artillery even as he joked that it “resulted in a lot of dead fish.” P.J. Crowley also said that the firing is unhelpful and that based on North Korea’s past actions “we’re likely to see more provocations.”

He said it is unclear what North Korea is “trying to achieve through this ongoing chest-thumping.”

North Korea has long sought a peace treaty and diplomatic relations with Washington to guarantee that the U.S., which has 28,500 troops in South Korea, would not invade and topple Kim Jong Il’s government. The U.S. has repeatedly said it has no intention of attacking the North.

Tension on the Korean peninsula is running high in the wake of the March sinking of the South Korean warship that an international investigation blamed on a North Korean torpedo attack. The North flatly denies the accusation and has warned any punishment would trigger war.

Tension deepened last week when South Korea launched large-scale naval training in response to the sinking, prompting its communist neighbor to warn it would counter the maneuvers with a “strong physical retaliation.”