Hundreds of Sudanese called for "revolution" on Monday, the second day of protests in support of four dead students originally from the conflict-plagued Darfur region.

Their deaths, following a crackdown on a tuition protest at Gezira University south of Khartoum, have sparked the largest outpouring of Arab Spring-style discontent since anti-regime protests in June and July.

The defense ministers of Sudan and South Sudan on Monday cut short talks on demilitarizing the tense border and other security issues but negotiations are going well, the South's minister said.

John Kong flew in from Juba on Sunday for talks with his Sudanese counterpart Abdelrahim Mohammed Hussein.

Six people were injured on Sunday as Sudanese police clashed with hundreds of protesters showing support for four dead students originally from the conflict-plagued Darfur region, Agence France Presse reported.

Many people were also detained by the security forces, and a bus owned by the Khartoum city government was torched, the reporter said.

At least 10 people have been killed after South Sudanese troops opened fire on demonstrators angry at officials moving the seat of local authority outside a state capital, the United Nations said Sunday.

"The SPLA (army) opened fire" on protesters "demonstrating the excessive use of force," said U.N. peacekeeping mission spokesman Liam McDowall.

Sudanese authorities on Saturday shut a university after four students, originally from the conflict-plagued Darfur region, were found dead following an alleged crackdown on a tuition protest.

The order coincided with the arrest of students who tried to demonstrate in Khartoum after holding a news conference about the deaths, which they blame on authorities and their "militia."

Two Iranian warships docked in Port Sudan on Saturday, a witness said, marking the second port call by the Iranian navy in Sudan in five weeks.

The Sudanese military said it was a "normal" port call but Israeli officials have expressed concern about arms smuggling through Sudan.

An outbreak of mosquito-borne yellow fever which has killed at least 165 people in Sudan's Darfur region is Africa's worst in decades, the World Health Organization said on Thursday.

"Definitely, this outbreak now could be classed as the largest outbreak" since at least 1990, WHO country representative Anshu Banerjee told AFP.

The only Darfur rebel group to have signed a peace deal with the Sudanese government on Thursday accused the army of launching a deadly attack which the authorities then lied about.

"Yesterday (Wednesday), Liberation and Justice Movement forces came under attack from SAF (Sudanese Armed Forces) near El Fasher," the rebels' chief Eltigani Seisi said in a statement to reporters.

A Sudanese military drone went down on Wednesday in Khartoum's twin city of Omdurman, the official SUNA news agency said, but there was no apparent damage or casualties on the ground.

"A pilotless military plane has gone down" inside Omdurman, a heavily-populated area, SUNA said in a brief SMS alert which gave no further details.

At least 30 people were killed and dozens wounded in a cattle raid between warring communities in South Sudan, the latest in a string of bloody clashes between rival ethnic groups, officials said on Wednesday.

Youths armed with automatic rifles from the Dinka Gok ethnic group in Lakes state marched for around eight hours to attack a rival group of cattle herders from the Dinka Rek tribe in Warrap state, said local official Isaack Mayom Malek.