U.S. Ambassador to Japan John Roos traveled to Nagasaki on Sunday for the first time since he assumed his post in August last year, becoming the fourth top U.S. envoy to do so, according to Nagasaki Prefecture.

After arriving at the Nagasaki Atomic Bomb Museum with his wife, Susan, Roos shook hands with Nagasaki Mayor Tomihisa Taue and museum curator Tomoo Kurokawa before viewing exhibits related to the 1945 atomic bombing of the city by the United States in World War II.

"Our visit today has reinforced for me the importance of President (Barack) Obama's resolve to work with all nations towards the peace and security of a world without nuclear weapons," Roos wrote in the museum's comments book for visitors.

Taue asked Roos to convey a request for Obama to visit Nagasaki.

After leaving the museum, Roos went to the monument marking ground zero to offer flowers. He was expected to meet Nagasaki Gov. Hodo Nakamura on Monday.

On Aug. 6, Roos attended a ceremony to commemorate the 65th anniversary of the atomic bombing of Hiroshima in 1945, becoming the first U.S. representative to do so. But, citing scheduling conflicts, he skipped a similar memorial service on Aug. 9 in Nagasaki, which was devastated by an atomic bomb three days after Hiroshima.

Before the ceremony in Hiroshima, Roos phoned Taue in July, informing the mayor of his wish to visit Nagasaki.

The last time a U.S. ambassador went to Nagasaki was in 1994, when Walter Mondale visited.