African Union troops are overstretched in Darfur

The US called the emergency meeting of the council, saying the letter was "a direct challenge".

However, there is no agreement about how to react to Sudan's warning. Some members say it should be ignored.

UN chief Kofi Annan said Darfur, which has endured a three-year conflict, was "on the brink of catastrophe".

A 7,000-strong African Union (AU) force has failed to end the conflict - in which 200,000 people have died and two million people have fled their homes.

'Hostile act'

The president of the Security Council for October, Japanese Ambassador Kenzo Oshima, said some members felt "the language [in the letter] was inappropriate and offensive".

Mr Oshima said a resolution condemning the letter had been considered, but it was decided to concentrate on how to restore peace in the troubled region.

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Sudan's letter said that contributing to a proposed UN peacekeeping force would be seen as "a hostile act".

Sudan does not want the UN to take control of the peacekeeping force from the AU, saying that would be an attack on its sovereignty.

The council has approved plans to send a 20,000-strong force with a tough mandate but says it will only do so if Sudan agrees.

US envoy John Bolton said it demanded "a strong response".

Mr Bolton added that if Khartoum was allowed "to intimidate troop-contributing countries", this would mean the failure of UN plans to deploy a robust UN force in Darfur.

But Britain's ambassador to the UN, Emyr Jones Parry, said the letter was "unacceptable", but that it would be best to ignore it.

"In my view... the best thing one can do with it is to actually just quietly assume it hadn't been sent," he told the BBC.

The council said it was seeking "clarification" of the letter from Sudan's UN ambassador.

'Catastrophe'

Washington says Sudan's military is helping carry out a genocide against Darfur's black African residents.

But Sudan denies backing the Arab Janjaweed militias, accused of riding into villages on horses and camels, killing, raping and looting.

Instead of reconciliation and building of trust, we are witnessing intensified violence and deeper polarisation

Kofi Annan

UN Secretary General

Sudan says the suffering in the region is being exaggerated for political reasons.

Delivering his report to the UN on Thursday, Mr Annan said humanitarian access to Darfur was at its lowest level since 2004.

He said a peace deal agreed in May had had little effect.

"Instead of reconciliation and building of trust, we are witnessing intensified violence and deeper polarisation. The region is again on the brink of a catastrophic situation," Mr Annan said.

Mr Jones Parry spelled out a four-point strategy to ease the suffering in Darfur: