(Adds comments from UN official)
By Amy Sawitta Lefevre
BANGKOK, May 27 (Reuters) - More than 2,500 migrants could still be stranded on boats in the Bay of Bengal and Andaman Sea, according to estimates by the United Nations, as Thailand prepares to host a regional meeting it said was focused on "immediate action" to tackle the crisis.
Thousands of Rohingya Muslims from Myanmar and migrants from Bangladesh have tried to land in Thailand, Malaysia and Indonesia since a Thai crackdown on people smugglers in early May led to trafficker crews abandoning them at sea.
Regional governments have struggled to respond, although images of desperate people crammed aboard overloaded boats with little food or water prompted Indonesia and Malaysia to soften their initial reluctance to allow the migrants to come ashore.
More than seven boats carrying around 2,600 people are thought to be still at sea, according to data from UNHCR and the International Organization for Migration (IOM) sources.
Friday's meeting in Bangkok will bring together 17 countries from the Association of Southeast Asian Nations (ASEAN) and elsewhere in Asia, along with the United States, Switzerland and international organizations.
"The meeting focuses on immediate actions to tackle the issue," said Panote Preechyanud at the Department of Information at the Thai Ministry of Foreign Affairs on Wednesday.
"It is an urgent call for the region to comprehensively work together to address the unprecedented increase of irregular migration across the Bay of Bengal in recent years."
The gathering takes place against the grim backdrop of Malaysia's discovery of nearly 140 shallow graves at 28 suspected people smuggling camps strung along its northern border, some of which authorities believe were abandoned in haste when the Thai crackdown began.
On Wednesday, Malaysia said it had detained 12 policemen for suspected links to the trafficking gangs.
Thailand, under pressure from the United States to do more to combat people smuggling, began its crackdown after finding at least 36 bodies in similar graves just across the border.
That made it too risky for smugglers to bring their human cargo ashore. Since then, more than 3,000 migrants left to fend for themselves have landed in Malaysia and Indonesia.
Southern Thailand and northern Malaysia are part of a well-trodden route for people smugglers transporting Rohingya Muslims, who say they are fleeing persecution in Myanmar, and Bangladeshis escaping poverty at home.
The UNHCR, the UN's refugee agency, cautioned that its figures for those still risking sickness and starvation at sea were loose estimates.
Malaysia, which says it has already taken 120,000 illegal immigrants from Myanmar, and Indonesia said last week they would give temporary shelter to those migrants already at sea, but that the international community must shoulder the burden of resettling them.
Thailand has refused to allow the boats to land, saying it is already sheltering 100,000 migrants from Myanmar, but has deployed a naval task force to offer medical aid at sea.
The United States has said the deadly pattern of migration across the Bay of Bengal would continue unless Myanmar ends discrimination against the Rohingya, a mostly stateless minority of 1.1 million people who live in apartheid-like conditions, mostly in the western state of Rakhine.
Myanmar denies the Rohingya face persecution and says it is not the source of the problem, suggesting many of the "boat people" are economic migrants from Bangladesh.
Vijay Nambiar, the UN Secretary-General's special adviser on Myanmar, said upcoming elections in the country, likely to be in November, could be a complicating factor in making attempts to resolve the Rohingya issue.
"My feeling is that the government is trying to address these problems," he told Reuters in the Rakhine capital of Sittwe.
"Very often they cannot do it too directly because of the upcoming elections, the need to curb excitement from some places, the need to curb violence. I don't think that finger-pointing is going to deliver."
The UNHCR, IOM and UN Office on Drugs and Crime - the three agencies invited to the regional meeting - have shared a 10-point action plan with regional governments, including a proposal to address the issue of citizenship which is a problem for the Rohingya in both Myanmar and Bangladesh.
Some diplomats were skeptical, however, about how much would be achieved in a region where collective action can be stifled by ASEAN's policy of non-interference in other member's affairs.
"Most people are hinging on what happens in Bangkok for a solution - short-term and long-term," said one Western diplomat in the region.
"There's not going to be a solution next week. If there's more recognition of the problem, then more countries may be willing to help. The long-term solution has to be in Myanmar. It is an ASEAN issue and has to be resolved by ASEAN." (Additional reporting by Al-Zaquan Amer Hamzah in Kuala Lumpur and Antoni Slodkowski in Sittwe; Editing by Raju Gopalakrishnan and Alex Richardson)