Seoul and Washington have started talks on extending the range of South Korea's missiles to help deter North Korea, news reports said Wednesday.

The allies have been in negotiations since late last year to revise a defence accord which imposes a maximum range of 300 kilometres (187 miles) on such missiles, Yonhap news agency and the Chosun Ilbo newspaper said.

"The two countries share the view that the range of our missiles should be extended," Yonhap quoted a government source as saying. "It's necessary to extend the range to 1,000 km or longer."

A defence ministry spokesman declined to comment.

Following a North Korean rocket launch in April 2009, the then-South Korean Prime Minister Han Seung-Soo told parliament the bilateral missile accord should be amended to extend the agreed maximum range.

But the issue was not raised at a bilateral security meeting in October 2009, with Washington concerned about an arms race in Northeast Asia, Yonhap said.

The need to bolster South Korea's missile capability took on new urgency when North Korea allegedly sank a South Korean warship, killing 46 sailors, in March last year.

It shelled a frontline island in November, killing four South Koreans.

South Korea believes the North has 1,000 missiles of various types, many of them targeted at Seoul or other locations in the South.

These include medium-range missiles capable of travelling more than 3,000 km and able to hit US bases in Japan and Guam.

The North has also test-fired three intercontinental ballistic missiles, most recently in April 2009 when one flew over Japan and landed in the Pacific.

During a regional tour last week, US Defense Secretary Robert Gates predicted the North would develop an intercontinental missile capable of reaching his country within five years.

The two Koreas have been technically at war since their 1950-1953 conflict ended only in an armistice. A total of 28,500 American troops are stationed in the South.