"If the United Nations is serious about its engagement with Sudan, it should tell this man to suspend what he is doing with this so-called indictment. There will be grave repercussions."
The BBC's Laura Trevelyan at UN headquarters says there is considerable alarm about the political consequences of such a move.
Reuters news agency reports that UN staff in Khartoum have been sent an urgent note, urging them to "upgrade their personal security measures" and avoid travel at the weekend and after dark.
"Ensure that you have an adequate supply of food and water in your home for several days, and that your vehicle is fully fuelled. Keep your personal documents and other essentials available," it says.
A UN aid worker in Khartoum, who did not want to be identified, told the BBC that issuing an arrest warrant against Mr Bashir would only frustrate peacekeeping efforts.
"The president has sworn to fight to the last man and will not surrender to the ICC," he said.
Following news of the ICC's intentions, ambassadors in Khartoum were summoned to the foreign ministry where they were told that an indictment would badly affect the stability of the whole region, state media reports.
Sudan analyst Juliet Flint told the BBC that if Mr Bashir was indicted, it would be at an "inopportune moment".
She warned that Mr Bashir is unlikely to react "coolly and calmly" and could clamp down on aid workers and peacekeepers in Darfur.
It is estimated that two-thirds of Darfur's surviving population rely on humanitarian assistance.
On Tuesday, seven Darfur peacekeepers died and 22 others were injured, seven critically, in an ambush - one of the deadliest assaults on UN forces in recent years.
UN Secretary General Ban Ki-moon condemns the attack on UN peacekeepers
The UN says its peacekeepers fought for over two hours to repulse suspected Janjaweed fighters, who were armed with mortars and rocket-propelled grenades.
The Sudanese government blamed rebel groups for the attack.
The UN estimates that some 300,000 people have died because of the Darfur conflict but Sudan's government says the scale of the violence has been exaggerated.
The United Nations and African Union have a joint peacekeeping force in Darfur. But it has just 9,000 of the planned 26,000 troops and has been struggling to contain the violence.
Ms Flint said an indictment would be "laudable in theory, but this would be an initiative fraught with risk for the people of Sudan".
"There is no doubt that the war in Darfur was orchestrated at the highest level of government," she said.
She said Mr Bashir was recently humiliated by Darfuri rebels who attacked a city across the river from the capital in May.
"A wounded animal would strike back," she said.
The ICC accuses the entire Sudanese government of war crimes in Darfur, although the government denies organising the Janjaweed.
The ICC has already issued arrest warrants for a minister and a militia leader on 51 counts of war crimes and crimes against humanity.
There is concern that the fragile Darfur peace process could be further weakened, along with the shaky peace in South Sudan, where a separate civil war ended in 2005.
There are also UN peacekeepers in South Sudan.
Sudan does not recognise the ICC and has refused to hand over a government minister and militia leader accused of war crimes last year.
Ali Kushayb, a leader of the Janjaweed militia, and Ahmad Harun, Sudan's current humanitarian affairs minister have both denied involvement in war crimes.
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