WASHINGTON (AFP) – United States President Barack Obama is set to meet with the leader of South Korea, who is seeking security guarantees as a standoff escalates with nuclear-armed North Korea.

The summit comes a day after the latest show of defiance by North Korea, which said that some 100,000 people rallied to denounce a tightening of United Nations sanctions on the hardline communist state for testing a nuclear bomb.

South Korean President Lee Myung-Bak has indicated that he wants Obama, who has set a goal of abolishing nuclear weapons, to reiterate that South Korea is under the US security umbrella.

Secretary of Defense Robert Gates told Lee in a meeting Monday that the United States was committed to defend South Korea "through all necessary means, including the nuclear umbrella," Lee's office said in a statement.

As the meeting between the heads of states takes place, Japanese officials have announced that the country is set to toughen sanctions on the hardline Stalinist state of North Korea to punish the communist regime for its latest nuclear and missile tests.

The Japanese government was widely expected to ban all exports to the impoverished state, after the ruling Liberal Democratic Party (LDP) backed the move on Monday.

"We are in the final stage of adjusting policy on additional measures against North Korea, including measures on exports... and financial transactions," said top government spokesman Takeo Kawamura.

"We will announce the plan as soon as we make the decision."

As part of its overseas military operations, the United States stations some 28,500 troops in South Korea and a further 40,000 more in nearby Japan, which has tense relations with Pyongyang.

South Korea's Lee, a conservative businessman, took over last year and – delighting many in Washington – reversed a decade-long "sunshine policy" under which South Korea put few restrictions on aid to the impoverished North.

The US Congress greeted him by passing a resolution demanding that North Korea end its "hostile rhetoric" against Lee, routinely berated in Pyongyang's state media as "the traitor."

"I think it's important that the president and the secretary of state know that Congress will stand behind them if they have to take stronger action," said the resolution's main sponsor, Republican Congressman Peter King.

"I think everything should be on the table."