North Korea's Supreme Court on Sunday sentenced U.S. citizen Matthew Miller to six years' hard labor for "hostile" acts, two weeks after he and two other detained Americans had pleaded for help from Washington.

Miller becomes the second American serving a hard labor prison term in the North amid accusations that Pyongyang is using them to extract political concessions from Washington.

The United States urged North Korea on Sunday to let three Americans held in the reclusive country return home, after Pyongyang said one of them will stand trial this week.

Matthew Miller, who is being held in North Korea along with fellow U.S. citizens Kenneth Bae and Jeffrey Fowle, will go on trial on September 14, state media said earlier Sunday.

North Korea will put a detained U.S. citizen on trial on September 14, state media said Sunday, less than a week after Matthew Miller made a highly unusual televised plea for help from Washington.

A top North Korean diplomat left Saturday for a tour of Europe amid signs reclusive Pyongyang has started to adopt a more active approach to foreign relations, a report said.

Kang Sok-Ju, a secretary of the central committee of the ruling Workers' Party, is scheduled to visit Belgium, Switzerland and other European countries, Japan's Kyodo news agency reported from the North Korean capital, quoting diplomatic sources.

North Korea on Saturday test-fired three short-range missiles off its east coast, putting the South's military on alert ahead of a traditional harvest holiday.

The missiles, launched from a location near the southeastern port of Wonsan early Saturday, flew 210 kilometers (131 miles) before splashing down in the Sea of Japan (East Sea), the South Korean military joint chiefs of staff said.

North Korea's reactor at its main nuclear site, capable of giving the isolated regime plutonium for nuclear weapons, may be operational again, the U.N. atomic watchdog said in a report seen by AFP Friday.

"Since late August 2013, the agency has observed, through analysis of satellite imagery, steam discharges and the outflow of cooling water" at the reactor at Yongbyon, the International Atomic Energy Agency said.

North Korea said Friday it would send home a South Korean man who entered the country illegally in a rare case of a defection to the impoverished communist state.

The man, identified as Kim Sang-Gun, was "intercepted" after entering the North illegally through a third country, North Korea's Red Cross Society said according to state-run news agency KCNA.

South Korean police and military officials have launched a joint investigation into a letter threatening to "execute" the country's defense minister, officials said Friday.

South Korea said Thursday it would create a combined army unit with the United States, reportedly tasked with destroying North Korea's weapons of mass destruction in the event of an all-out conflict.

The mechanized unit led by a U.S. major general will be set up in the first half of next year, the South's defense ministry said, as part of elaborate preparations for any future war between the two Koreas.

Three U.S. citizens held in North Korea pleaded Monday for help to secure their release, urging Washington to send an envoy to the hardline communist country.

As government minders looked on, the three, who appeared tense, also said they had been well treated in the autocratic state.