SEOUL - Seven people were injured after shells landed on a civilian area of South Korea Thursday, according to Yonhap news agency and broadcaster YTN.

Several shells fell on seven civilian houses in the inter-Korean border town of Pocheon, some 40 km north of the capital Seoul, at about 10:05 am local time.

Four people were seriously wounded and three others suffered minor injuries.

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An investigation was underway into where the shells came from.

Earlier in the day, South Korea and the United States held a combined live-fire exercise in Pocheon, mobilizing more than 160 pieces of military hardware, such as tanks, self-propelled howitzers, helicopters and stealth jets.