JOHANNESBURG, Jan 16 (Reuters) - The African Union's credibility and its ability to tackle disputes on the continent will be severely battered if Sudan President Omar al-Bashir is elected AU chairman, human rights groups said on Monday.

African diplomats and analysts say Sudan is poised to be elected to the rotating chair of the AU when heads of state and government hold a summit in Khartoum next week, replacing Nigerian President Olusegun Obasanjo.

Diplomats told Reuters Bashir appeared to have secured the vital support of South African President Thabo Mbeki, a key player in the AU's Peace and Security Council. Mbeki is also head of an AU panel overseeing Sudan's reconstruction after the end of that country's two-decade conflict between southern rebels and the government in Khartoum.

There was no immediate comment from the office of the South African president, who was in Liberia on Monday to attend the inauguration of President Ellen Johnson-Sirleaf.

In a letter to Mbeki, Obasanjo and other African leaders, 50 human rights groups and non-governmental organisations (NGOs) said in Sudan's case, Bashir could not be entrusted with the task of driving peace moves in Darfur, in western Sudan.

Among the groups that signed the letter was the Third World Network -- Africa, Southern Africa Non-Governmental Organisations Network, the Media Institute of Southern Africa, African Foundation for Development and the African Youth Parliament.

"We wish to express our deep concern with respect to the ongoing plans by the African heads of state and government to confer the AU Presidency for the year 2006-2007 on Sudan; and in particular to President Omar al-Bashir," the groups said.

"We seriously believe that such an action will deeply undermine and erode the credibility of the AU and at the same time compromise the authority of its institutions," they said after a weekend meeting in the Kenyan capital Nairobi.

MAJOR COUP

After pulling off a major coup in hosting the summit, Sudan's prestige will soar if it wins the chair. Neighbouring Chad -- which accuses Sudan of backing rebels seeking to overthrow President Idriss Deby -- is the only African nation openly campaigning against Khartoum.

Analysts say ideally Sudan would not be allowed to chair the African Union but the AU does not have clear guidelines on who should occupy the post, making it difficult to block Sudan.

"The result of this dilemma is that the chair will not be credible and Sudan will not be able to continue with urgency on subjects that have occupied Obasanjo's chairmanship, that is peace in Ivory Coast and resolving the Darfur crisis," said Siphamandla Zondi, Africa director at Johannesburg think-tank Institute for Global Dialogue.

The AU has a 7,000-strong mission monitoring a shaky ceasefire in Darfur, so a Sudan chairmanship of the pan-African body would put the government in a bizarre situation.

"It would be awkward, pretty much unprecedented," said UK-based analyst Patrick Smith, of Africa Confidential newsletter.

But there is a tradition among African leaders not to interfere in disputes within a fellow country's borders.

"It is going to be difficult for the AU heads of state and government not to allow Sudan to chair," said Prince Mashele, of the Institute for Security Studies in Pretoria. "African leaders are diplomatic in dealing with sensitive issues and will not raise Sudan's rights or governance issues publicly."

DARFUR RECORD

The NGO letter said the human rights and humanitarian situation in Sudan's Darfur region was one of the worst in the world. It said Bashir's government was seen as partly responsible for the crisis that has seen tens of thousands of people killed and 2 million uprooted since February 2003.

The letter said Khartoum and its allied "Janjaweed" militia still held civilians hostage, blocked access to parts of Darfur and obstructed the work of relief agencies. Sudan undermined many AU agreements to bring peace to Darfur, the groups added.