The rival governments of north and south Sudan signed an accord Monday to demilitarize the disputed Abyei region and let in an Ethiopian peacekeeping force, mediator Thabo Mbeki told the UN Security Council. The north\'s troops who occupied Abyei on May 21 will withdraw under the deal, said the former South African president who leads an African Union panel. Amid escalating tensions ahead of southern Sudan\'s formal declaration of independence on July 9, the north has also launched a major operation in neighboring South Kordofan province and the United States said it had \"worrying\" evidence of attacks that could be crimes against humanity. Tens of thousands of people have already fled Abyei, mainly to the south. Aid agencies say there is a growing humanitarian crisis in South Kordofan. Mbeki has been negotiating an accord between the two sides for weeks. He said the accord was signed \"in the last hour\" before he spoke to the Security Council by video link from Addis Ababa. \"We are encouraged by this news,\" said Susan Rice, UN ambassador for the United States which has been closely involved in efforts to bring the two sides together. Rice said it was very urgent for the Ethiopian troops to be deployed as quickly as possible. UN special representative Haile Menkerios said that the United Nations \"will be ready to assist a quick deployment of these troops to Abyei\" as soon as the Security Council gives approval. Abyei has been mainly inhabited by Ngok Dinka people who consider themselves southerners. But Misseriya nomads from the north herd their cattle through the territory in the dry season and are strongly supported by the Khartoum government. Two decades of civil war up to 2005 left two million dead and the rivalry has escalated again ahead of the formal split with serious fighting now reported in South Kordofan, where more than 70,000 are reported to have fled their homes. Up to 10,000 people are said to have sought refuge at the UN compound in Kadugli. US ambassador Rice welcomed the announcement of the north-south peace accord but said Sudan remains a threat to international peace. She said there were \"horrifying\" reports of the fighting in South Kordofan\'s main city, Kadugli, where she said aerial bombing threatened civilians and UN workers. Rice said forces loyal to the Khartoum government had sought out and \"allegedly executed\" sympathizers of the southern Sudan in Kadugli. She said there were unconfirmed reports that the north was arming its followers and placing mines in areas believed to support the south and that politicians, lawyers and doctors had been detained. All these could be \"crimes against humanity\", Rice said. Mbeki said that senior political leaders from South Kordofan and neighboring states had started arriving in Addis Ababa and that from Tuesday there would be \"serious\" talks on ending the fighting there. \"We are hoping to move to that South Kordofan matter immediately and urgently,\" he said. The AU envoy said there had also been progress in talks on relations between the north and south after the July 9 split. He said discussions had been \"finalized\" on issues such as dividing Sudan\'s international debt and on oil revenues, currency and other economic measures. He said a package had been put to both sides for approval.