North Korea will gain "nothing" by making threats, U.S. President Barack Obama said Friday, warning it of sanctions with "more bite" if it went ahead with a fourth nuclear test.

Speaking in South Korea as satellite images revealed the North could be preparing for an underground explosion, Obama stressed that Washington and Seoul stood "shoulder to shoulder" in their refusal to accept a nuclear North Korea.

German Chancellor Angela Merkel will visit U.S. President Barack Obama next week to discuss international security, trade and other issues, Berlin said Friday.

"What now worries all of us, the very serious situation in eastern Ukraine, will be a central focus, as well as how to react to it," said Merkel's spokesman Steffen Seibert, also citing transatlantic economic and trade issues.

U.S. President Barack Obama said Friday that a push for unity with Hamas by Palestinian leader Mahmud Abbas was "unhelpful" but vowed America would not give up trying to broker peace in the Middle East.

The proposed tie-up between Abbas's Fatah faction and the radical Islamist group earlier prompted Israeli Prime Minister Benjamin Netanyahu to halt peace talks on the grounds Hamas wanted to "liquidate" Israel.

U.S. President Barack Obama will meet representatives of a range of civil-society groups -- including some which have harshly criticized Malaysia's government -- when he visits the country this weekend, activists said.

Obama aims to strengthen ties with Muslim-majority Malaysia when he arrives Saturday for a two-night stay, the first visit by a serving U.S. president since 1966.

Barack Obama may have enthused publicly about the sushi he was served at an exclusive Michelin-starred restaurant, but he stopped eating half way through the meal, reports said Thursday.

Pictures showed the U.S. president being served sake by Japanese Prime Minister Shinzo Abe at the counter of the tiny Tokyo eatery, as part of a meal billed as a chance for bonding, at which neither wore a tie.

President Barack Obama said Thursday Russia was not abiding by the letter or the spirit of the Geneva deal to ease tensions in Ukraine, and said new sanctions against Moscow were already teed up.

The president weighed into an increasingly bitter U.S.-Russian dispute over the actions of separatist militias in the east of the country Washington says are backed by the Kremlin, during his tour of Asia.

U.S. President Barack Obama arrived in Tokyo Wednesday to launch an Asian tour dedicated to reinvigorating his policy of "re-balancing" U.S. foreign policy towards a dynamic Asia.

Obama landed aboard Air Force One to begin a state visit to Japan, which comes as regional tensions boil over maritime territorial disputes and fears that North Korea could soon carry out a new nuclear test.

U.S. President Barack Obama will reportedly dine at a tiny Tokyo sushi restaurant on Wednesday -- a place with three coveted Michelin stars but only a handful of seats -- ruled with an iron rod by its redoubtable 88-year-old owner, Jiro.

The world's most powerful man, who will be dining with Japanese Prime Minister Shinzo Abe, will be entirely at the mercy of Jiro Ono, whose exacting standards and tireless work ethic have made him legendary among global foodies.

The islands at the center of a corrosive row between Tokyo and Beijing are covered by the U.S.-Japan defense alliance, Barack Obama told a newspaper ahead of his arrival in Tokyo Wednesday.

Obama, who begins a tour of Asia that will also take in South Korea, the Philippines and Malaysia, is the first sitting U.S. president to explicitly affirm that hostile action against the island chain would spark an American reaction.

President Barack Obama departed Washington Tuesday for a weeklong tour of Asia in what the White House has called a "rebalancing" eastward of U.S. foreign policy.

Obama left Andrews Air Force Base at 10:30 am (1430 GMT) aboard Air Force One, on a trip that was first to make a stopover in Washington state to visit the scene of a deadly landslide.