South Korean President Lee Myung-Bak promised "flexibility" Monday in his North Korea policy, in a possible sign of a softer approach towards Pyongyang aimed at easing longstanding tensions.

Lee made the comments a day before he leaves for Washington for a summit later this week with President Barack Obama.

"The government will exercise flexibility for the peaceful resolution of North Korea's nuclear problem while pursuing principled dialogue," Lee told parliament in a budget speech read by his prime minister.

He also offered the North humanitarian aid.

In August Lee appointed Yu Woo-Ik as unification minister in charge of cross-border ties, replacing a hardliner. Yu has also promised "flexibility" but the president is not thought to be have used the term before.

"The government will make efforts to put inter-Korean relations on a normal footing and continue to prepare for peaceful unification," Lee said.

Relations between the Korean neighbours soured after Lee took office in early 2008 and linked major aid to progress on the North's nuclear disarmament, a policy which enraged Pyongyang.

Ties turned icy after Seoul accused Pyongyang of torpedoing a warship in March 2010 with the loss of 46 lives.

The North denied involvement but hostility worsened after it killed four South Koreans in a bombardment of a frontier island last November.

Lately there have been signs of a slight thaw, with the South's government allowing religious groups and a senior ruling party politician to visit the North.

Last month the two sides held a second round of talks designed to pave the way for a resumption of six-nation negotiations on the North's nuclear programme.

US and North Korean officials met separately in New York in late July.

A senior Seoul diplomat quoted by Yonhap new agency said Monday that Washington and Pyongyang would hold a second bilateral meeting soon.

"The second round of talks between the US and North Korea will be held by the end of this month, at the latest," the diplomat said on condition of anonymity, adding they may take place in a third country.

The North abandoned the six-party nuclear talks in April 2009 and staged its second nuclear test a month later. It now calls for an unconditional resumption of the negotiations which also group China, Japan and Russia.

The US and South Korea say the North must first take steps to show it is serious about the process, for example by shutting down its uranium enrichment programme.