Rebels in Sudan's Darfur region and in the troubled border states of Blue Nile and South Kordofan have formed an alliance to overthrow the government of President Omar al-Bashir, a statement released by the rebels said.
The alliance, called the Sudanese Revolutionary Front, is bent on "toppling the regime of the [Sudan's ruling] National Congress Party with all possible means" and replacing it with a democratic system, the groups said in the joint statement sent to the Reuters news agency on Saturday.
A joint military committee will be formed to co-ordinate military action against Khartoum, said the alliance, without elaborating.
"This is a military and political alliance. We will co-ordinate fighting to end this government which wants no peace," said Ibrahim el-Hilu, a spokesman for one faction of the Sudan Liberation Army, a rebel group in Darfur.
The declaration of war on Khartoum raised the prospect of more violence in the volatile areas.
It comes nearly a week after Bashir visited Kurmuk in Blue Nile after his troops seized the town from a branch of the Sudan People's Liberation Army North (SPLA-N).
A spokesman for Sudan's foreign ministry criticised the announcement of a rebel alliance [Al Jazeera]
Analysts say the move signalled attempts at closer military co-ordination among various rebel groups left in Sudan after South Sudan seceded in July under a 2005 peace agreement with Khartoum.
Khartoum has held several rounds of talks with rebels in the western region of Darfur to try to resolve the conflict, which began in 2003 after the main rebel group there - the Justice for Equality Movement - complained of marginalisation.
While there has been no serious fighting in Darfur, where the UN maintains a peacekeeping mission, fighting erupted between SPLA-N rebels and the Sudanese army in South Kordofan in June and spread to neighbouring Blue Nile state in September.
Khartoum accuses South Sudan of arming fighters in the two states and has taken the matter to the UN Security Council, but South Sudan rejects the charges.
The SPLA-N forces in Blue Nile and South Kordofan states served as the 9th and 10th divisions of the southern rebel forces called SPLA during the civil war between south and north, but the peace agreement placed the areas they fought for in the north.
Many SPLA-N fighters' uniforms still show the flag of the former rebel group that now governs South Sudan.
Obaid Murawah, a spokesman for the Sudanese foreign ministry, told Al Jazeera that the country was not looking for an excuse to invade its southern neighbour and that it was battling rebels within its own borders.
"Our [the government's] point of view is that if anyone is seeking power they should do so through the election boxes," he said.
Analysts say the move may mean no immediate military threat to Bashir but could dash hopes of a political solution to end insurgencies in Darfur and southern border regions.