SEOUL, Aug 28 (Reuters) - South Korea will drop the symbolic tag of "main enemy" for North Korea in its biannual defence white paper, an official said on Thursday, even though Pyongyang has threatened to reduce its neighbour "to ashes".

"Not using the term 'main enemy' does not mean that we changed our stance. North Korea is still a substantial and present threat," said Major Seo Young-suk, an official in the Defence Ministry's public affairs office.

Seo said the white paper, which is due out around December, is not based on political relations between the two Koreas but is an assessment of South Korea's general military situation.

North Korea bristled at the "enemy" tag when it was first used in a ministry paper about 13 years ago.

Left-leaning presidents who served from 1998 to 2008 had pushed the ministry to drop the term in order to help their policies of engaging the reclusive communist state. The ministry said it removed the term in papers in 2004 as well as 2006.

Ties between the two Koreas, technically still at war, have chilled since President Lee Myung-bak took office in February, saying Seoul would cast a cold eye on aid to the North and tie handouts to progress Pyongyang makes in nuclear disarmament.

The impoverished state, angered at the cut in what had been a free flow of aid, called Lee "a traitor to the nation" and said in late March that it was ready to reduce the South to ashes.

Earlier this week, North Korea rattled neighbours by saying it would stop disabling a Soviet-era plant it uses to make arms-grade plutonium despite requirements it do so in a nuclear disarmament deal it reached with five regional powers.

Tension between the two Koreas was raised again on Wednesday when the South said it had charged a North Korean woman with spying. It said the woman had posed as a defector in the South and had received military secrets in exchange for sexual favours.

South Korea has about 670,000 troops, who are supported by about 27,000 U.S. soldiers in the country. North Korea stations most of its 1.2 million troops near its border with the South.

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