Bangkok, Thailand — The United States should overcome the diffidence that has marked its behavior in Asia since the end of the Vietnam war, Southeast Asian allies told Secretary of State George Shultz in Bangkok this week.
And Washington should become more energetic politically in support of the anti-Vietnamese guerrilla coalition in Kampuchea and in the fight against world recession.
This was the message to Mr. Shultz during discussions between the Association of Southeast Asian Nations (ASEAN) and its Western allies. The association's 16 th foreign ministers' meeting was followed this week by meetings with the foreign ministers of Canada, Australia, New Zealand, Japan, West Germany, Denmark, and the United States.
On policy toward Kampuchea, the US has consistently maintained that it would follow ASEAN's lead. As Secretary Shultz told the ASEAN foreign ministers (representing Thailand, Malaysia, Singapore, Indonesia, and the Philippines): ''Regional security, like economic progress, is more solidly constructed if rooted in local initiative.''
ASEAN, however, feels this is too modest.
''We'd like to see a little more US effort on Kampuchea,'' said a senior ASEAN official.
''Washington shouldn't just take the lead from ASEAN,'' said another official , ''it should take its own initiatives.''
During the discussions, the Southeast Asian nations suggested a number of concrete steps the US could take to assist the Coalition Government of Democratic Kampuchea (CGDK), which has been resisting Vietnamese occupation of that country. These steps include:
* President Reagan should receive Prince Norodom Sihanouk as CGDK president, preferably with the honors of a head of state, when the prince goes to the US in September to defend the coalition's UN seat. ''The Sihanouk visit will be an acid test of US attitude,'' said an ASEAN official.
ASEAN hopes that a visit with Mr. Reagan would encourage US allies to receive Prince Sihanouk as well, thereby increasing the coalition's international standing. It also hopes that a personal appeal by the prince might dislodge some US aid for the coalition.
* The US should substantially increase its aid for the anti-Vietnamese coalition. So far, the US has given humanitarian aid, largely through private or international agencies working with Kampuchean refugees.
''Now we'd like the US to give non-humanitarian aid,'' an ASEAN official said. ''If the US hasn't got out of the Vietnam syndrome, it can start with non-lethal weapons - communications equipment, boots, uniforms.'' The official said that his US counterparts agreed to consider this. ''They're obviously wondering if they can get it through Congress,'' said the official.
(A senior US official traveling with Mr. Shultz denied that the subject of aid for the coalition had been raised in official meetings. This was confirmed by ASEAN diplomats who said that because of the issue's sensitivity, the question was raised unofficially.)
Representatives of ASEAN countries and other participants in the discussions implied that the US had been secretly giving financial support to the coalition, particularly the faction led by Son Sann, for some time.
''The coalition needs money to survive,'' said an ASEAN official. ''And it's clear that they have been surviving quite well. . . .'' The official hinted very broadly that this had been partly thanks to US government money. ''Of course,'' the official added with a grin, ''if they are doing it, they would not like it publicized.''
Speaking personally, an American diplomat doubted that the US would want to revert to the traditional Asian role envisaged for it by ASEAN. He pointed out that this might be a mixed blessing for its allies. ''After all,'' he said, ''we're a lot of different things to different people out here. Perhaps we've drawn the right conclusions from Vietnam'' (namely, that direct involvement was not necessarily the most effective way of supporting one's allies).
A similar reserve was visible in Secretary Shultz's opinions on the world economy.
Speaking on behalf of ASEAN during talks with the US, the Philippine Foreign Minister, Gen. Carlos Romulo, mentioned ASEAN's concern at events in Belgrade, where the sixth United Nations Conference on Trade and Development (UNCTAD) is under way. General Romulo also expressed disquiet at the effects of world recession.
Developing countries, he claimed, had lost $600 billion when commodity prices collapsed in 1981-82. Unemployment, hunger, and disease, he told Shultz, had been the cost of these economic problems. The West with its stronger economies, Romulo said, should bear a great share of responsibility in the world economic recovery.
Interest rates should be held down and protectionism limited. And the West should give more financial help, ''including official development aid,'' said General Romulo. And, he added, world recession should not just be attributed to market forces, but also to the ''underlying inequitable sharing of economic burdens and benefits.''
Shultz replied that the US could help the developing world by recovering its own economic strength. A strong US economy could then help developing countries by providing private investment, he said. And a healthy US economy would provide a better market for Southeast Asia's products.
Some ASEAN officials seemed pleased with Mr. Shultz's reference to possible greater access to US markets. But some of the poorer ASEAN countries appeared unhappy: ''Shultz's approach is a trickle down one,'' said a senior ASEAN ambassador. '' 'We get well, you get well.' His lack of attention to the North-South dialogue is frankly depressing.''