N. Korea Holds Live-Fire Drill on Maritime Border

North Korea held a brief live-fire drill near its maritime border with South Korea on Tuesday, hours after U.S. President Barack Obama wrapped up an Asia trip that Pyongyang had denounced as provocative.

A similar drill one month ago saw a number of shells land on the South Korean side of the border, prompting a response that resulted in both countries firing hundreds of artillery rounds into each others territorial waters.

There was no repeat this time around, with Seoul warning it would respond "strongly" to any border violation.

North Korea had given advance notice of the drill which began around 2:00 pm (0500 GMT), according to the South Korean defense ministry.

"It lasted around 10 minutes and 50 shells were fired. None landed on the South side," a ministry spokesman said.

Residents on the South Korean border islands of Yeonpyeong and Baengnyeong were advised to move to civilian shelters, while South Korea scrambled at least four fighter jets to patrol the area.

Used to repeated North Korean provocations, some simply brushed off the drill as an irritation.

"I'm so sick of it," said one inn owner on Baengnyeong island.

"The whole thing is just scaring my customers away. I'm more likely to die from business losses than an attack from the North," she told Agence France Presse by telephone.

The drill came amid concerns that the North is preparing to conduct its fourth atomic detonation, with recent satellite images showing stepped-up activity at its main nuclear test site.

It also followed the visit to Seoul last week by U.S. President Barack Obama as part of a four-nation Asian tour that ended in Manila earlier Tuesday.

- Angered by Obama -

Obama had angered Pyongyang by demanding that the North abandon its nuclear weapons program and by threatening tougher sanctions if it went ahead with another test.

"North Korea's continued pursuit of nuclear weapons is a path that leads only to more isolation," Obama told American troops based in Seoul.

"It's not a sign of strength. Anybody can make threats. Anyone can move an army. Anyone can show off a missile," he said.

North Korea denounced Obama's visit as "dangerously" provocative and said it had only reaffirmed Pyongyang's policy of preparing to fight "a full-scale nuclear war".

North Korea has conducted three nuclear tests, the most recent -- and most powerful -- in February last year.

The de-facto maritime boundary between the two Koreas -- the Northern Limit Line -- is not recognized by Pyongyang, which argues it was unilaterally drawn by U.S.-led United Nations forces after the 1950-53 Korean War.

Both sides complain of frequent incursions by the other and there were limited naval clashes in 1999, 2002 and 2009.

In November 2010, North Korea shelled Yeonpyeong island, killing four South Koreans and briefly triggering concerns of a full-scale conflict.

Inter-Korean ties had seemed to be enjoying a thaw earlier this year when the North -- following rare, high-level official talks -- hosted the first reunion for more than three years of families separated by the Korean War.

But tensions began to escalate after the South launched its annual joint military exercises with the United States in late February.

The North was also angered by a U.N. report detailing Pyongyang's record of human rights abuses and by the U.N. Security Council's criticism after it test-fired two medium-range missiles in March.