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By Alexander Smith and Stella Kim

SEOUL, South Korea — South Korea fired warning shots at a North Korean drone after it crossed over their heavily militarized border Wednesday, according to officials in Seoul.

It was the latest in a series of tense exchanges between the rival neighbors since the North claimed to have tested a hydrogen bomb last week — an assertion treated with deep skepticism by the U.S. and others.

The drone flew dozens of yards south of the border before returning immediately following the warning shots, South Korea's Joint Chiefs of Staff spokesperson Jeon Ha-gyu told a news briefing.

The military fired around 20 machine gun rounds as a warning, the South's Yonhap news agency reported.

It is not the first time a North Korean drone has crossed the border. A primitive unnamed aircraft likened to a "model airplane" by experts crash landed in South Korea in 2014.

Jeon also said that North Korean propaganda leaflets had been found scattered over South Korea. The leaflets were likely floated over on balloons and described South Korean President Park Geun-hye and her government as "mad dogs," The Associated Press reported.

Related: 5 Things to Know About N. Korea's Nuclear Proclamation

The contretemps came as the U.S. House of Representatives overwhelmingly passed a bill Tuesday that would place tougher sanctions on North Korea following its claimed H-bomb test.

The bill would require the president to sanction anyone engaging with North Korea on weapons of mass destruction, arms, luxury goods, money laundering, counterfeiting and human rights abuses, plus those engaging in financial transactions to support North Korea.

South Korean President Park warned on Wednesday that the U.S. and its allies were working on inflicting "bone-numbing pain" on North Korea, according to Reuters.

Park also urged China, which is North Korea's most important ally, to rein in its pariah neighbor.

The Koreas are technically still at war after an armistice ended the Korean War in 1953.

The Associated Press and Reuters contributed.