KHARTOUM, Oct 12 (Reuters) - Security has worsened in Sudan's Darfur region with new attacks on refugee camps and indiscriminate banditry hindering aid access to 650,000 people, U.N. officials said on Wednesday.

U.N. spokeswoman Radhia Achouri said two-thirds of South Darfur state was a no-go area for U.N. staff after clashes between rebels and government and recent attacks on African Union troops and aid workers there.

In West Darfur state, U.N. staff cannot leave the main town el-Geneina because of attacks on the roads there.

"Throughout the region, thousands of civilians are fleeing to IDP (internally displaced people) camps as a result of the recent violence," she told reporters.

"The issue of banditry, looting is taking quite serious proportions," Achouri said, adding the United Nations had lost count of the number of attacks by bandits in Darfur. "The situation is quite serious," she added.

The African Union reported an attack by Arab militia on a refugee camp last month that killed 34 people.

The AU itself, with a 6,000-strong force in Darfur monitoring a shaky ceasefire, was targeted by rebels in the past week, suffering its first casualties with five killed and 38 taken hostage.

In West Darfur, Achouri said staff and aid were travelling by air, but that this was not sustainable in the long term. South and West Darfur contain a majority of the almost 2 million people who have fled their homes during the 2-1/2 year conflict in the remote region bordering Chad.

Niels Scott, a U.N. humanitarian official, said the restrictions made aid access very problematic and difficult. He said there were 300,000 refugees in West Darfur and 350,000 in South Darfur affected by the security situation.

There are almost 11,000 aid workers in Darfur.

Achouri said the United Nations was concerned at the resurgence in the past month of attacks by Arab militias on villages, as well as renewed clashes between rebel and government forces, despite ongoing peace talks in the Nigerian capital Abuja.

"In all these incidents of banditry there is use of force ... guns so we have casualties, people being beaten up ... people being killed sometimes and some women raped also or abducted so it's quite a phenomenon," she added.

Non-Arab rebels took up arms against Khartoum in early 2003 accusing it of neglect and of monopolising power and wealth. Khartoum responded by arming mostly Arab militias, who now stand accused of a widespread campaign of looting, killing and burning in non-Arab villages.

Tens of thousands have been killed by the violence, which the United States called genocide. Khartoum denies genocide but the International Criminal Court (ICC) is investigating accusations of war crimes in Darfur.