Insecurity forces aid cuts in Sudan's Darfur - U.N.

* U.N. 'very concerned' at increasingly insecure environment

* Surge in kidnaps of aid workers and peacekeepers

KHARTOUM, June 24 (Reuters) - Worsening security in Sudan's Darfur region is forcing aid agencies to cut or suspend some relief programmes, the United Nations said on Thursday.

The statement came two days after armed raiders abducted two German men working with humanitarian groups in the capital of South Darfur state, Nyala, and three days after attackers shot dead three Rwandan peacekeepers in the centre of the region.

In an unusually frank statement from inside Sudan, the U.N. mission called on Sudanese authorities to make a "concerted effort" to arrest and prosecute people who attack, abduct and rob humanitarian workers.

"The U.N. and humanitarian partners are very concerned at the increasingly insecure environment in Darfur ... particularly in the past two months," said the unsigned statement.

"The reduction of access due to insecurity has already resulted in some cases of either a complete suspension or a serious reduction of activities and delivery of assistance by humanitarian agencies."

The U.N.'s World Food Programme this month said more than 400,000 Darfuris might miss out on food aid in June because aid convoys were struggling to get to them.