The US is mulling over a decision to put North Korea back on its list of states harboring terror, while Seoul unveiled plans to counter a possible attack by Pyongyang.

US Secretary of State Hillary Clinton said Sunday that several senators had asked President Barack Obama to put North Korea back on its list of countries that sponsor terrorism, from which it was removed in October 2008.

"Well, we're going to look at it…There's a process for it. Obviously we would want to see recent evidence of their support for international terrorism," she told ABC television.

The recent US threat follows Obama's pledge on Saturday to take a "very hard look" at tougher measures against "defiant" North Korea.

Obama said that "North Korea's actions over the last couple of months have been extraordinarily provocative", adding that Washington was not "intending to continue a policy of rewarding provocation".

The US president was alluding to nuclear and short-range missile tests conducted by the North last month, which drew international outrage, especially from neighboring South Korea and the US.

Reinstating North Korea on the watch list would re-activate sanctions against the country -- which was lifted in October last year.

Also on Sunday, South Korea's Joint Chiefs of Staff (JCS) briefed President Lee Myung-Bak on plans for a huge counter-attack on North Korea should Pyongyang fire missiles at Seoul's navy ships.

"North Korea's firing of ground-to-ship missiles at our navy ships would prompt counter-attacks simultaneously from surface, air and sea," JCS chairman Kim Tae-Young had told Lee, according to AFP.

The contingency plan, drafted amid growing cross-border tensions, was released as more than 600,000 South Korean soldiers, backed by 28,500 US troops, reportedly deployed on the Korean peninsula, near the tense sea border in the Yellow Sea.

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