By Andrew Harding BBC East Africa correspondent, Nairobi

Five weeks of peace talks aimed at ending Sudan's 19 year long war have started in neighbouring Kenya.

It has been billed as Sudan's best chance for peace in years.

You can still talk and fight at the same time

SPLA spokesman Samson Kwaje

And the Sudanese government delegation has described the talks as a defining moment and praised American help in creating a new momentum for peace.

But the southern rebel delegation is sounding less optimistic about the chances of negotiating a lasting settlement to end a war that has killed more than two million people.

The delegations have five weeks to make a breakthrough

The talks began in a gloomy Nairobi conference room, with the Kenyan mediator scolding both sides for showing a lack of commitment and flexibility in the past.

The Sudanese government delegation replied that years had been wasted, but that now was a decisive moment; the government was ready to negotiate constructively.

Sitting opposite, the rebel SPLA delegation sounded sceptical.

It rejected calls for an immediate cessation of hostilities, arguing that could only come after a political settlement.

Both sides are now under huge international pressure to end their civil war.

Obstacles

The United States has taken a lead, helping to negotiate a ceasefire on one front line in the Nuba mountains.

The rebels have made recent gains

But the obstacles to a lasting peace are daunting; the southern rebels represent a mainly animist and Christian population, which is deeply suspicious of the Islamic government in the north.

The rebels want Sudan to become a secular state, something the government rejects.

The rebels also refuse to rule out building a fully independent state in the south.

The government is determined to preserve Sudan's unity.

International mediators have been trying to find inventive compromises.

Constitutional reforms and federal structures which might satisfy both sides.

Those proposals will be looked at closely over the next month.

Oil

But it is still easy to be sceptical about these negotiations.

The delegations have five weeks to make a breakthrough

There have been so many failures in the past.

And things have been made even more complicated by the discovery of vast oil reserves along the front lines.

The government talks about sharing the oil revenues with the whole of Sudan.

The rebels suspect the north wants all the profits.

The oil seems more likely to keep fuelling Sudan's war, than to benefit its people.