South Korean police on Saturday stopped a planned launch of anti-North Korean leaflets across the tense border, sparking an angry protest from activists, witnesses said.

A group of defectors from the North had said they would float 200,000 leaflets over the heavily militarized frontier by balloon on Saturday to protest Pyongyang's dire human rights record.

The United States on Thursday called for the "immediate release" of a Korean-American tour operator sentenced in North Korea to 15 years of hard labor for "hostile acts" against the government.

"We urge the DPRK authorities to grant Mr Bae amnesty," deputy acting State Department spokesman Patrick Ventrell told journalists, referring to jailed US citizen Pae Jun-Ho, who is known in the United States as Kenneth Bae.

Former U.S. president Jimmy Carter has no plans to visit North Korea, his spokeswoman said Thursday, after a report that the Nobel laureate intended to go with hopes of freeing a detained American.

"President Carter has not had an invitation to visit North Korea and has no plans to visit," spokeswoman Deanna Congileo said.

North Korea could be ready within weeks to start operating a light-water reactor that has triggered growing concern amid the regime's vows to build more nuclear weapons, researchers said Wednesday.

The U.S.-Korea Institute at Johns Hopkins University said that satellite pictures taken in March and April appeared to show that North Korea was doing final work inside the reactor and cleaning up after completing construction.

South Korea's government will provide more than $270 million in emergency loans to help companies affected by the shutdown of a jointly-run factory park in North Korea.

The finance ministry said Thursday the 300 billion won ($273 million) in relief funds will help cover debts and operating costs of around 120 South Korean companies that were forced early last month to halt production at factories in the Kaesong industrial complex amid high tensions on the Korean peninsula.

An American detained for nearly six months in North Korea has been sentenced to 15 years of labor for crimes against the state, the North's state media said Thursday, a development that further complicates already strained ties between Pyongyang and Washington.

The sentencing of Kenneth Bae, described by friends as a devout Christian and a tour operator, comes amid signs of tentative diplomacy following weeks of rising tensions in the region. North Korea had been warning of nuclear war and missile strikes, an angry response to U.N. sanctions for conducting a long-range rocket launch in December and a nuclear test in February, as well as U.S.-South Korean military drills in South Korea.

The United States and South Korea on Tuesday wrapped up military drills at the center of soaring tensions with North Korea, as Pyongyang ignored a new overture over a flagship joint industrial zone.

The two-month-long "Foal Eagle" air, ground and naval field training exercise -- which involved more than 10,000 U.S. troops along with a far higher number of South Korean personnel -- had infuriated Pyongyang.

Islands at the center of a territorial row between Japan and China are covered by a military protection accord between Washington and Tokyo, U.S. Defense Secretary Chuck Hagel said Monday.

"The United States does not take a position on the ultimate sovereignty of the islands, but we do recognize they are under the administration of Japan and fall under our security treaty obligations," Hagel said at a news conference with his Japanese counterpart Itsunori Onodera.

North Korea appears to be gearing up for a major military exercise, a report said on Sunday, amid tensions over an expected missile test and South Korea's pullout from a joint industrial complex.

Preparations are under way near the North's western port of Nampo for a combined live-fire drill involving artillery units and air force jets, the South's Yonhap news agency said, citing a Seoul government source.

A limousine carrying Japan's Prime Minister Shinzo Abe was involved in a five-car pile-up at a toll gate in Tokyo on Saturday but he escaped without any injuries, police said.

Two guards in a police car accompanying Abe's official vehicle suffered slight injuries to the face, a Metropolitan Police Department spokesman said.