US Vice President Joe Biden on Tuesday praised Japan's courage and resolve as he visited its tsunami-shattered coast, where American forces helped with a large-scale relief effort.

Biden, near the end of his Asia tour, is the top-ranking American official to travel to the disaster zone where the March 11 quake and tsunami claimed more than 20,000 lives and sparked the Fukushima nuclear disaster.

"I am honoured and truly humbled to have an opportunity to visit this place, to see so much devastation and tragedy," said Biden, speaking at the airport of Sendai, one of the cities hardest-hit by the disaster.

The American vice president said that Japan's response to what he called "this God-awful tragedy" had "demonstrated for the world to see so much heroism, courage and resolve and selflessness".

"I came to express not only my commitment to say, 'We will do whatever we can to help,' but to tell you how much the president, how much I, how much the American people admire your character."

Walking through the devastated landscape later, Biden saw a house that was reduced to a shell amid knocked-down pine trees and debris. He laid white flowers on a pile of rocks before observing a moment's silence.

The United States, which has maintained bases across Japan since World War II, mobilised more than 20,000 troops and some 160 aircraft in disaster relief and recovery operations after Japan's worst peace-time catastrophe.

One of the core achievements of the US "Operation Tomodachi" (Friend) was to clear Sendai's international airport, where the tsunami had swept aircraft, cars, mud and debris across runways and into terminals.

Japan's outgoing centre-left Prime Minister Naoto Kan earlier thanked Biden for his country's "enormous assistance" and said he would like to "reiterate our gratitude" on behalf of Japan's people.

"You do not need to express gratitude to us," Biden replied. "You'd do the same for us. Our only regret is that we could not do even more."

The meeting was clouded by the fact that Kan, under fire for his post-disaster leadership, has only days left in office before he is set to make way next week for Japan's sixth new premier in five years.

Biden told reporters after their hour-long talk that "Japan will rebound, and be stronger -- literally stronger -- than before the devastation."

The US tsunami aid effort has helped rebuild relations which were long strained by a dispute over a US airbase on Japan's Okinawa island.

Many residents of the far-southern island have long chafed under a heavy post-war US military presence and demanded that the contentious base, the Marine Corps Air Station Futenma, be moved off Okinawa.

Kan's predecessor Yukio Hatoyama stepped down last year after first promising to move the base off the island, then reneging on the pledge -- managing to anger both Washington and Okinawans in the process.

The allies have since agreed to go ahead with a planned move of the base within Okinawa island, but have been forced to scrap a 2014 deadline for the shift.

Defence planners in both countries see Okinawa's US bases as significant at a time when China is building up its naval forces and showing an increasing assertiveness in territorial disputes in nearby waters.

Biden spoke about his visit to China and pointed at the half-century security alliance between Tokyo and Washington.

"We are a Pacific power. You are a Pacific power. We are allies, both economically and politically," Biden told Kan. "It's something that we value a great deal."

Biden, who arrived in Tokyo Monday night after touring China and Mongolia, on Wednesday visits the US military headquarters in Japan at the Yokota airbase west of Tokyo.

He used his five-day visit to China, his first as vice president, to reassure senior leaders about the safety of US Treasuries following a historic downgrade this month of the country's top-notch credit rating.

Returning to the theme in his talk with Kan, Biden said: "There are voices in the world who are counting us out. They are making a very bad bet."