Mr Biden met South Korean leader Park Geun-hye, a close US ally
US Vice-President Joe Biden has urged Japan and South Korea to "improve their relations and co-operation".
Mr Biden was in Seoul on the third leg of an Asian tour dominated by tensions over China's newly-declared air defence zone.
China's zone covers disputed islands controlled by Japan and an area claimed by Seoul.
Mr Biden said on Thursday China's move had "caused significant apprehension in the region."
Mr Biden discussed the zone with South Korean leader Park Geun-hye. The issue of North Korea and ways to restart long-stalled nuclear disarmament talks were also high on the agenda.
As the meeting with Ms Park began, Mr Biden emphasised the US commitment to a strategic shift to the region.
"I want to make one thing absolutely clear: President Obama's decision to rebalance to the Pacific basin is not in question," he said. "The United States never says anything it does not do."
In a speech later, Mr Biden said: "The entire region will be more stable and more secure if the leading democracies - Japan, South Korea and the US - are able to improve their relations and co-operation with one another."
Japan occupied Korea before and during World War II, and tensions remain between Tokyo and Seoul that have recently been exacerbated by rows over a separate disputed island and lingering historical issues.
'Do not accept'
Mr Biden arrived in Seoul on Thursday from Beijing, where he held what he described as "very direct" talks with President Xi Jinping on the new air defence identification zone (ADIZ).
The White House has described the establishment of the ADIZ as "unacceptable", but has urged Beijing not to implement it - rather than calling for it to roll back its move.
"We, the United States, do not recognise and we do not accept it, and will not change the way the United States conducts military operations in the region," White House spokesman Jay Carney said on Thursday.
The zone covers East China Sea islands that Japan, China and Taiwan claim. The islands, which are controlled by Japan, are called Senkaku in Japan and Diaoyu in China.