GENEVA - Sudan's human rights situation is grim with killings of civilians by government and rebel forces and arbitrary arrests and torture for political reasons, a United Nations investigator said on Monday.

Sima Samar, in a report for the U.N. Human Rights Council, said breaches of humanitarian law were being committed not only in the long-troubled Darfur region in the west but also in other parts of the country, including the south.

"Despite some steps by the government of Sudan, principally in the area of law reform, the human rights situation on the ground remains grim, with many interlocutors even reporting an overall deterioration," she wrote.

Samar, a former deputy prime minister of Afghanistan, said Sudanese government forces had attacked civilians by land and air in Darfur and other serious incidents had occurred in fighting between Darfuri rebel groups.

Serious rights violations had also occured when one insurgent group in Darfur attacked the city of Omdurman in May, and in fighting the same month between the Sudanese national armed forces and troops of the government of south Sudan.

Similar abuses had occurred in clashes between troops of the south, whose dominant Sudan People's Liberation Movement (SPLM) is part of a coalition government in Khartoum, and villagers in the Eastern Equatorial region, Samar said.

The SPLM joined a coalition national unity government with the north's dominant National Congress Party under a 2005 peace deal ending a long civil war, but relations between the two sides have remained strained.

Samar's report, to be discussed in the 47-member Rights Council on Tuesday, said it was essential that impartial, transparent and comprehensive investigations be held into all allegations of rights abuses.

She said concerns were mounting over violations not only of general human rights but also of civil and political rights in different parts of the country as preparations for general elections get under way.

"There have been widespread allegations of arbitrary arrests and detention, torture, incommunicado detention and serious violations of the right to a fair trial" since the start of the year, she said.

Samar said one of her principal concerns about the situation in Sudan, for which she has been special investigator for the U.N. since 2005, was impunity, or the failure to punish people responsible for rights abuse.