By Aung Hla Tun

YANGON (Reuters) - Myanmar's detained Nobel laureate, Aung San Suu Kyi, will support U.S. plans to engage with the isolated nation but only if opposition groups are involved in any dialogue, her party said on Friday.

The pro-democracy leader also offered to cooperate with the country's military rulers to seek an end to sanctions imposed by the United States and other countries, Nyan Win, spokesman for Suu Kyi's National League for Democracy (NLD) party, said.

The comments followed U.S. Secretary of State Hillary Clinton's announcement that Washington wanted dialogue with the country's rulers but would not lift its tight sanctions on them.

"(Suu Kyi) said she had always supported the idea of engagement. However, that engagement should be done with both the military government and the democratic forces," Nyan Win said.

He added in a separate statement that Suu Kyi had signed a letter on Friday to Myanmar's top military leader, Than Shwe, asking that she be allowed to work with the junta in trying to persuade countries to lift sanctions on the former Burma.

Speaking in New York on Wednesday, Clinton did not elaborate on the engagement plans, giving no indication about a timeframe, who would lead talks and what demands would be made in order for sanctions to be lifted.

The United States has imposed sanctions on Myanmar since 1988, when an estimated 3,000 people were killed in an army crackdown on pro-democracy activists.

U.S. relations with Myanmar appear to be less frosty than in recent years and last month's visit by U.S. Senator Jim Webb -- the first by a senior U.S. official in more than a decade -- was hailed by the junta as a big success.

Clinton said in July that the United States would help Myanmar if its army rulers held free, fair and inclusive elections and released Suu Kyi, who has been in detention, or "protective custody," for 14 of the past 20 years.

POSITIVE STEP

Analysts said the change in approach was a positive step that could eventually lead to reforms in a country crippled by five decades of economic mismanagement and oppressive army rule.

"The regime wants to normalize relations with the U.S. to convince the world and the Burmese people that their elections will be legitimate," said Win Min, a Burmese exile and lecturer at Payap University in Chiang Mai, Thailand.

"This engagement will be a gradual process. We shouldn't expect too much, but it's a lot better than the previous policy."

Suu Kyi was given another 18 months under house arrest last month for letting an American intruder stay at her home for two nights. Critics said that ruling was designed to keep her out of elections next year, the first since 1990.

Myanmar analyst Aung Naing Oo said dialogue would be advantageous for both sides. Engagement could eventually bring big benefits to Myanmar, while allowing the United States to gain a strategic foothold in a country traditionally allied with its powerful neighbor China.

"What's most significant is that there's a willingness from both sides. They know they have to talk and they know they need to build trust," said Aung Naing Oo.

"Although it's a positive and pragmatic move, the biggest stumbling block will be the negotiations. For this to work, there needs to be patience, compromise and a real understanding of the mindset of both parties."

(Writing and additional reporting by Martin Petty; Editing by Jason Szep)