Sudan's President Omar al-Bashir, has launched a major military offensive in southern Sudan, ordering his army to act without restraint.

Peace talks in the Kenyan town of Machakos collapsed on Monday after the rebels seized a key garrison town.

All of this leaves a real question mark over the commitment of both sides to the Machakos peace agreement

Ishbel Matheson, BBC

Only six weeks ago, both sides reached a framework deal aimed at ending Sudan's 19-year civil war.

The rebel Sudan People's Liberation Army (SPLA) is fighting for greater autonomy for the Christian and animist south from the Arab and Muslim dominated government in Khartoum.

The United States has pressured both sides to end the war and says it is "deeply disappointed" by the government's pull-out.

But US Assistant Secretary of State for African Affairs Walter Kansteiner said he remained optimistic:

"I still think there is enough of a commitment from both parties for talks to make real progress."

'Serious blow'

The BBC's Ishbel Matheson says that the government is furious at the rebels' audacious seizure of the town of Torit at the weekend.

Mr Bashir was quoted by a government-owned newspaper as declaring a general mobilisation of his forces.

I think there is a heavy dose of gamesmanship on both sides

Stephen Morrison African analyst

"The armed forces have been directed to move in all directions until Torit, Kapoeta and other areas are recaptured," Al-Anbaa newspaper quoted him as saying.

Our correspondent says the fall of Torit is a serious blow to the government, as it is left with just one garrison town, Juba, in the south.

She says that the government, as well as the rebels, has been on the offensive.

Still hopeful

Khartoum has seized strategic positions around the oil fields in western Upper Nile.

"All of this leaves a real question mark over the commitment of both sides to the Machakos peace agreement," she says.

Both delegations have now left Machakos but analysts say it is not necessarily the end of the peace talks.

"I think there is a heavy dose of gamesmanship on both sides. I wouldn't jump to the conclusion that the peace process is over," Stephen Morrison, Africa programme director at Washington's Center for Strategic and International Studies, told Reuters news agency.