NEW YORK (CNN) -- The number of deaths in Sudan's Darfur region since 2006 may have been underestimated by as much as 50 percent, the U.N. undersecretary-general for humanitarian affairs said Tuesday.

An man injured in clashes with tribal fighters outside the African Union Mission in southern Darfur in 2007.

In March, international figures, including U.N. data, put the death toll in Darfur at 200,000, with another 2.5 million people displaced.

But 300,000 are believed to have died in the tribal conflict in the past two years, said John Holmes, who also is the United Nations emergency relief coordinator.

Holmes said that sexual violence has increased and that food allotments for civilians affected by the civil war will be halved in a few days.

Holmes gave the U.N. Security Council an update on conditions in the western Sudan region, revisiting a report he gave a year ago.

"I am sad to say that the humanitarian situation remains as grim today as it was then, if not more so," he said.

In 2008, 100,000 civilians have fled their homes, many not for the first time.

The Darfur conflict began five years ago, when ethnic African tribesmen took up arms, complaining of decades of neglect and discrimination by the Sudanese government.

Sudan's Arab-dominated government is accused of responding by unleashing tribal militias known as Janjaweed, which are said to have committed the worst atrocities against Darfur's local communities.

There also have been intra-rebel and tribal clashes, Holmes added.

He blamed the military arm of the Justice and Equality Movement faction of the Sudan Liberation Movement for the most recent round of violence, and he accused the government of using "disproportionate force" and failing to differentiate between civilians and rebels in their response.

"Darfur today is still characterized by insecurity, lawlessness and impunity," he said. "A particularly worrying feature is evidence of high levels of sexual violence and exploitation in the northern corridor of west Darfur over the past two months."

This is shown by the increased number of women and girls seeking treatment after sexual brutality, Holmes said.

The World Food Programme announced last week that it will have to cut back its food distributions in May, partly because of attacks on convoys that reduced the amount of supplies they can get through, Holmes said.

Six aid workers have been killed this year, and 42 humanitarian posts have been attacked.

The supply line also is hurt by soaring food prices: The price of staples such as millet has doubled since April 2007, Holmes said.

While expressing gratitude to the Sudanese government for improved cooperation under various agreements, Holmes said there still is no physical access to internally displaced persons and some other groups.

He said he was "saddened and angry" that after five years, there has been no lasting solution to the suffering.

He made several recommendations to the government, including disbanding the Janjaweed, providing security for citizens, ending impunity for criminals and shouldering more of the financial burden for humanitarian aid.

Holmes also said rebels must stop their attacks against people and humanitarian convoys. E-mail to a friend

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