South Korea says it exchanged fire with North Korea at border By K.J. Kwon and Jethro Mullen, CNN Updated 10:25 AM ET, Thu August 20, 2015 | Video Source: CNN

(CNN)In an escalation of the tense situation in the region, North and South Korea exchanged fire over their heavily fortified border Thursday, the South Korean Defense Ministry said.

South Korea detected a projectile, assumed to be a small rocket, that was fired toward the western province of Gyeonggi, a Defense Ministry official told CNN.

The South Korean military responded by firing a few dozen shells at the area from which the North Korean projectile was fired, the official said.

The Pentagon is monitoring the escalating tension closely, Cmdr. William Urban, a Defense Department spokesman, told CNN’s Brian Todd.

And a U.S. official told CNN’s Barbara Starr that the U.S. believes that North Korea fired a shot at a South Korean loudspeaker, and South Korea responded with 36 artillery shells.

The U.S. believes North Korea deliberately placed mines in the path of a South Korean patrol in the demilitarized zone between the two countries, sparking the exchanges, the official said.

Tensions spiked on the Korean Peninsula after two South Korean soldiers were seriously wounded by landmines on August 4 in the demilitarized zone.

South Korea also has accused the North of planting the mines, an allegation that Pyongyang denies.

Seoul vowed a “harsh” response to the landmines and resumed blaring propaganda messages over the border from huge loudspeakers.

The move infuriated North Korea, which called the broadcasting “a direct action of declaring a war.” It threatened over the weekend to blow up the South Korean speakers and also warned of “indiscriminate strikes.”

North Korea pumped its own propaganda broadcasts over the border Monday, the same day South Korea started military exercises with the United States and other countries. Pyongyang says it views the drills as a prelude to an invasion.

The two sides have exchanged fire in the past at sensitive points. In November 2010, North Korea shelled an island near the countries’ disputed maritime border, killing two South Korean marines.

They also traded fire in October 2014. A clash took place between patrol boats in the Yellow Sea, and then another flared days later over land after North Korean gunners apparently targeted balloons carrying leaflets critical of the country’s reclusive regime.