Rebels in Sudan's Darfur region and troubled southern border states said Saturday they had formed an alliance to topple the government of President Omar Hassan al-Bashir, raising the prospect of more violence in the volatile areas.

Sudan accused South Sudan, which split away as an independent country in July, of having helped set up the alliance and called it an act of aggression.

Analysts said the new alliance showed closer coordination among various rebel groups left in Sudan after the South seceded under the terms of a 2005 peace agreement.

Sudan's army is fighting separate insurgencies in the western region of Darfur as well as in the southern states of South Kordofan and Blue Nile bordering South Sudan.

Violence in the joint border region has led to tensions between Khartoum and South Sudan. The United Nations accused Sudan this week of having bombed a refugee camp in South Sudan, a charge denied by Khartoum.

Khartoum and Juba accuse each other of backing rebels in each other's territories.

Darfur's main rebel groups -- the Justice and Equality Movement (JEM) and the Sudan Liberation Army (SLA) -- and the SPLM-N, which fights the army in South Kordofan and Blue Nile, said they had formed a political and military alliance.

The alliance called Sudanese Revolutionary Front is focussed on toppling the regime of the (Sudan's ruling) National Congress Party with all possible means and replace it with a democratic system, the groups said in a joint statement sent to Reuters Saturday.

A joint military committee will be formed to coordinate military action against Khartoum, it said, without elaborating.

This is a military and political alliance. We will coordinate fighting to end this government which wants no peace, said Ibrahim el-Hilu, a spokesman for one faction of the SLA.

Sudan's security services accused South Sudan of having helped form the alliance by allowing rebels to meet in the new country to prepare their declaration, the state-linked Sudanese Media Centre (SMC) said on its website.

The sponsoring of South Sudan's government of the so-called Sudanese Revolutionary Front is a clear sign of aggression of the southern state against Sudan, SMC quoted a security official as saying.

Analysts say the alliance may mean no immediate military threat to Bashir but dashes hopes of a political resolution to insurgencies in Darfur and southern border regions.

Fighting erupted between SPLM-N rebels and the army in South Kordofan in June and spread to neighbouring Blue Nile state in September. Both states are home to populations who sided with the South Sudan during a decades-long civil war with the Khartoum government and now complain of marginalisation.

A separate insurgency has raged in Darfur since 2003, again involving rebel groups who say they have been marginalised by the political elite in Khartoum.

Sudan signed a peace accord with a small Darfur rebel group Thursday, but JEM and other larger groups have refused to sign.

(Reporting by Ulf Laessing and Khalid Abdelaziz)