Britain called on Chad on Saturday to arrest Sudanese President Omar al-Bashir while he visits N'Djamena for a regional summit and to hand him over to the International Criminal Court.

Chad is one of the countries signed up to the ICC's founding treaty which are legally bound to arrest Bashir, who is wanted for war crimes, crimes against humanity and genocide in the long-running Darfur conflict.

The United States on Friday warned of a dangerous increase in tensions on the undemarcated border between Sudan and South Sudan after reports of clashes between the two sides.

Washington is "deeply concerned" by the reported incidents in disputed border areas between South Sudan's Upper Nile state and Sudan's Blue Nile, the U.S. embassy in Khartoum said.

Aid delivery is in jeopardy for an estimated 100,000 people affected by violence in Sudan's Darfur region unless authorities grant better access, the United Nations warned on Thursday.

One humanitarian agency has said the number of displaced people is the largest in recent years in Darfur, where a decade of civil war has been compounded by inter-Arab violence, banditry and tribal fighting.

Khartoum-backed militia in Sudan's South Kordofan state have killed 17 civilians, rebels said on Saturday, adding to tensions with South Sudan over a failure to implement security agreements.

The rebels accused a group of ethnic South Sudanese of ambushing a civilian lorry on Friday at Abu Nuwara, about 80 kilometers (50 miles) from the border with South Sudan's Upper Nile state.

Islamists fleeing advancing French forces in Mali have been spotted in Sudan's troubled Darfur region, a Sudanese rebel commander told Agence France Presse on Friday.

"We saw them," said Abdel Wahid Mohammed al-Nur, commander of a Sudan Liberation Army (SLA) faction which is fighting Khartoum's troops in Darfur.

The head of the Palestinian Islamic Jihad, accused by Israel of receiving arms which transit through Sudan, has met President Omar al-Bashir in Khartoum, official media reported.

Bashir discussed "issues of mutual concern" with Ramadan Abdullah Shallah during talks on Sunday at the presidential guest house, the state SUNA news agency said. It gave no further details.

The South Sudanese army on Sunday accused Sudan of launching a deadly air assault along their volatile border, but Khartoum promptly denied the claims.

A spokesman for the South Sudanese army (SPLA) said the attack occurred on its soil along the border in Upper Nile state.

Animal carcasses are rotting and tens of thousands of people have sought shelter in schools and government offices in Sudan's Darfur region after violence this month, the U.N. said on Thursday.

About 100,000 people are newly estimated to have been displaced or severely affected across a wide area of the Jebel Amir district of North Darfur state, the United Nations humanitarian agency (OCHA) said in its weekly bulletin.

Amnesty International said Wednesday that Sudanese security officers were reportedly involved in attacks over gold mining that killed up to 200 people, in one of the Darfur region's worst recent outbreaks of violence.

The unrest displaced more than 70,000 people, the U.N. has said, citing government figures.

At least five people were killed in South Sudan's troubled Jonglei state in clashes between government troops and rebel soldiers who had come for peace talks, officials said Monday.

Hundreds of civilians fled the clashes which broke out Sunday -- during which the market in the remote town of Pibor was set on fire -- as a government delegation arrived to negotiate with the breakaway commander of a local rebel movement.