Seoul, South Korea (CNN) Tensions between North Korea and South Korea escalated on Wednesday after South Korea said an unidentified aircraft approached the demilitarized zone.

After issuing warning broadcasts, military forces from the South responded by firing warning shots with machine-gun fire, an official with the South Korean Defense Ministry said. The unidentified North Korean aircraft "immediately went towards the North," he said.

It was not immediately clear whether the drone was armed or what its mission was.

However, a former U.S. Army senior intelligence officer who served there suggested the purpose was either surveillance or instigation by North Korea.

"It's very likely these were trying to figure out troop disposition," said retired Lt. Col. Tony Shaffer. "The second thing they were trying to do was probably provocation."

'This is no small issue'

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Shaffer warned that miscalculation by any of the forces arrayed on either side of the heavily fortified border, even at the individual level, could result in a larger expanded conflict, one that could endanger the thousands of American military personnel stationed there.

"This is no small issue," he said.

Cmdr. William Urban, Defense Department spokesman, said American forces along the DMZ are closely monitoring the situation.

"We are concerned that additional North Korean provocations could heighten tensions, lead to a cycle of escalation," he said.

The incident came just days after South Korea began blaring music and propaganda from loudspeakers along the border in retaliation for a nuclear test conducted last week by the North.

"I think this is quite a dangerous situation," said Bonnie Glaser of the Center for Strategic and International Studies. "There could be firing by the North Koreans, which then could prompt South Koreans to retaliate very quickly," she said. "There's always a danger that things could escalate on the Korean Peninsula, lives being lost."

Propaganda leaflets distributed

In an additional provocation, South Korean soldiers on Wednesday found anti-South Korea and anti-U.S. propaganda leaflets, believed to have been sent by North Korea between Tuesday and Wednesday. The leaflets were found in Seoul, Uijeongbu, Dongducheon, Paju and other border cities, South Korean Defense Ministry officials told CNN.

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One of the things the colored leaflets said was, "Let's beat up Park Geun-hye's clique like you would do to a mad dog."

For her part, South Korean President Park Geun-hye said Wednesday said she was working with the United States and the United Nations to develop "painful and effective sanction measures against North Korea" in response to the test.

In the United States, members of Congress held a hearing to review ways to press North Korea to dial back its nuclear ambitions, with proposals ranging from providing South Korea with terminal high altitude area defense missiles to pushing China to cut off North Korea's access to Chinese banks.

North Korea defends its nuclear ambitions

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But North Korean leader Kim Jong Un on Wednesday defended his nuclear program, saying his country needs the capability to strike the United States if provoked.

At a ceremony in which he honored the scientists who carried out last week's controversial weapons test, Kim blamed the United States and its allies for "bringing dark clouds of a nuclear war" to the Korean Peninsula, according to his official website

He called for increasing the quantity and quality of North Korea's nuclear force, "capable of making nuclear strikes at the U.S.-led imperialists any time and in any space," the statement said, "if they encroach upon the sovereignty of North Korea and make threatening provocations."

North Korea brought international condemnation on itself last week after claiming to have detonated a hydrogen bomb, although experts doubted the atomic test reached the thermonuclear level.

Questions of Pyongyang's claims

It is not the only claim from Pyongyang over the past week that experts have questioned. North Korean state television last Friday showed an allegedly successful ballistic missile test launch from a submarine, with Kim looking on, but analysts are casting doubt both on whether it launched successfully and on whether it was launched from submarine.

Researchers with the Center for Nonproliferation Studies at Middlebury said that the heavily edited film seemed to show black smoke, several fireballs and even possibly falling debris.

The attempt "appears to have ended in failure," wrote analyst Catherine Dill, and in the final two frames of the clip, "the rocket appears to explode."

And a paper from analyst John Schilling with 38 North suggests the test was most likely from a barge , not a submarine.

Schilling notes that there is a support boat seen close to the launch location, which a barge would require, whereas a submarine would make it prohibitively dangerous. In addition, a satellite photo two days later shows the submarine docked next to a barge, and a crane that would remove or replace the launch canister is hovering over the barge, not the submarine.

And, he writes, "the North Koreans have edited their film in a way that suggests they are concealing a partial failure."