SEOUL - South Korea went on alert briefly Sunday after a North Korean coastal battery unit turned its radar on for firing amid tensions after a recent naval clash, a report said.

YTN television, quoting unnamed military sources, said the North's short activation of the radar sent the South rushing to relocate its 1,500-tonne destroyer out of range in the Yellow Sea.

The radar at a battery in Ongjin on the North's southwest coast was powered on for an hour until 2:00 pm (0500 GMT), YTN said, adding South Korea's naval patrol boats were on stand by for emergencies.

Seoul's defence ministry refused to confirm the YTN report in detail, but admitted having responded to unspecified movements by the North.

"There were some North Korean movements, and we took necessary steps. Everything has returned to normal now," a ministry spokesman told AFP without elaborating.

South Korea's military is already on high alert as the North threatened to take "merciless" action following a naval clash on November 10.

The clash in the Yellow Sea -- the first such skirmish in seven years -- left a North Korean patrol boat in flames, according to Seoul officials.

Sources quoted by local media said one North Korean sailor was killed and three wounded in the brief but intensive exchange of fire.

The South has since sent a destroyer to reinforce its military presence along the sea border, and two extra patrol boats.

It has also ordered army, navy and air force commanders in charge of border areas to step up surveillance and respond immediately to any provocation.

The North vowed Friday to take "merciless" military action to protect its Yellow Sea border with South Korea and warned that Seoul would pay dearly for any future naval clash.

The North refuses to recognise the sea border set by the United Nations after the 1950-1953 Korean War and demands that it be drawn further to the south.

The two Koreas had bloody naval skirmishes in 1999 and 2002 near the disputed maritime border.

The fresh tension came just days before US President Barack Obama visits Seoul for talks expected to focus on Pyongyang's nuclear programme.