By Adam Mynott BBC News, Nairobi

Heads of state and political leaders from Africa are gathering at the headquarters of the African Union (AU) in Ethiopia for a two-day summit beginning on Monday.

The situation in Darfur has been described as "catastrophic"

Climate change which threatens livelihoods all over Africa and a call for more dynamic scientific advances on the continent will occupy the public forum of the summit on Monday.

The coming year has also been designated by the AU as the International Year of African Football, and various events attended by football luminaries have been arranged.

But other issues will dominate the closed session of African leaders on Tuesday and much of the discussions which take place behind the scenes at the AU headquarters in the Ethiopian capital, Addis Ababa.

The ongoing Darfur crisis in western Sudan is not improving. In fact it is getting worse.

Tom Arnold, the head of the international agency Concern has just returned from Darfur.

"The situation is catastrophic," he said.

Urgent action

"The only way to turn it round is for urgent political action to improve security in the short term and to start a meaningful longer-term peace process."

The new UN Secretary General Ban Ki-moon has put Darfur at the top of his list of humanitarian priorities.

He is going to the AU summit to urge heads of government to press for more to be done to stop the killings and rapes, and to find a solution to help end the misery of the two-and-a-half million who are living in camps in poverty and fear.

His predecessor, Kofi Annan, thought he had secured the agreement of the Sudanese President Omar Hassan al-Bashir to allow UN troops to bolster the African Union peacekeeping mission in Darfur, but President Bashir continues to say that he will refuse to allow "neo-colonialist" UN forces into the western Sudanese province.

Omar al-Bashir's candidacy for AU chairman is splitting the group

Many African leaders accept that the AU on its own cannot bring the Darfur crisis to a close.

The other problem facing the African Union is what to do about Somalia.

It is still rocked by insecurity a month after the Islamic Courts Union which had controlled much of central and southern Somalia was defeated by the combined forces of Somalia's transitional government and Ethiopia.

The AU has committed itself to finding a "stabilisation force" of 8,000 soldiers to send into Somalia, but so far few African countries have come forward with offers of help. The longer it takes the worse it becomes.

Chairmanship row

Somalia and Darfur should be enough to fully occupy the combined diplomatic skills of all the leaders of Africa's 53 countries, but another matter threatens to overshadow the entire exercise.

A year ago Sudan was due to take over chairmanship of the AU but, because of Darfur, many African leaders felt it entirely inappropriate for a government accused of war crimes to be in charge of the pan-African body.

Somalia is fragile but few have offered peacekeepers

In part to avoid a total diplomatic breakdown, Sudan was, in effect, promised the role next time.

But Darfur is in an even worse state and many feel that it would be even more inappropriate for President Bashir to become chairman of the AU now.

This is likely to be very contentious, and has not been made any easier by the fact that one of the main rebel groups in Darfur, the Sudanese Liberation Army (SLA) saying that if Sudan is given the chairmanship then it will consider itself at war with the AU.