New infrastructure is changing millions of lives along the Mekong
Japan is hosting the leaders of South East Asia's five Mekong River nations in what analysts say is a move to counter growing Chinese influence.
Japan's Prime Minister Yukio Hatoyama is to meet his counterparts from Cambodia, Laos, Burma, Thailand and Vietnam for the two-day meeting.
Japanese officials said the meeting would also spur dialogue with Burma.
Mr Hatoyama has promoted the concept of an EU-style Asian community since taking office in September.
Much of the Mekong region has lagged in economic development for years, but China, Japan, the Asian Development Bank and others have stepped up investment in recent years.
"Narrowing the development gap is quite an important agenda point," said a Japanese foreign ministry official.
"Development of the Mekong region will be an important step towards building an East Asian Community," the official said.
He denied that Japan was competing with China for greater influence in the area, which is home to about 220 million people.
The Mekong region leaders will welcome Japanese investment
"We don't need to compete with others," the official said, arguing that Tokyo and Beijing have "very good relations" when it comes to co-ordinating policies on the region's development.
Despite this, the meeting does not include China, even though the Mekong flows through the country and China is actively pursuing investments in timber, rubber and mining in Laos, Cambodia and Burma.
Some analysts see the summit as a Japanese attempt to outflank China in the region.
Takashi Inoguchi, dean of the University of Niigata Prefecture, told AFP news agency that "the Japanese government thinks it is very important" to foster deeper ties with South East Asia in view of China's growing presence.
"The Japanese realise, they've realised for a long time, that they are being outmanoeuvred by the Chinese," Dr Richard Cronin, director of the South East Asia programme at the Henry L Stimson Centre, a think tank in Washington, told Reuters.
Japan is reportedly ready to offer billions of dollars in low interest loans to Mekong countries, and to pursue more roads and other infrastructure investments.
Talks are also expected to cover co-operation in programmes to protect the environment, combat climate change and offer training and education.
The Nikkei business daily said Mr Hatoyama would pledge 200bn yen ($2.2 bn; £1.3bn) in low-interest loans over three years.
The summit follows a Mekong foreign ministers' meeting in Cambodia in early October and is the first such meeting between Japan and the five nations, Japan's foreign ministry said.