US President Barack Obama wrapped up a historic trip to Asia, where he sought to deepen US involvement by boosting ties to former adversaries in the region.

He paid tribute to victims of the first atomic bomb in Hiroshima on Friday, the first American leader to visit the city devastated by the bomb that helped end World War II.

He also took part in a G7 summit in Japan, where he raised concern over escalating tensions with North Korea following a series of nuclear tests by Pyongyang.

Obama arrived there from Vietnam, where he formally reset the relationship with Washington's former foe by lifting an arms embargo, four decades after the US withdrew its last troops from the country.

He also appealed to its leaders to lift authoritarianism.

His visit came as China is rattling countries in the region with a series of reclamation and construction projects -- including airstrips -- on reefs and islets in disputed waters in the South China Sea.

Obama's trip provides a coda eight months before he steps down and almost five years after he announced Washington's "pivot" to Asia, a foreign policy strategy that has been overshadowed by emerging security threats from the Middle East and Russia.

Obama touched down at Andrews Air Force Base Friday evening before boarding helicopter Marine One to return to the White House.