UNITED NATIONS — Deploying all 26,000 members of a peacekeeping force in conflict-wracked Darfur will take many more months because of growing insecurity and logistical difficulties, U.N. Secretary-General Ban Ki-moon said in a report circulated Friday.

Even when the joint United Nations-African Union peacekeeping force is fully deployed, he said, the only way to end the fighting that has killed up to 300,000 people and forced 2.5 million to flee their homes will be through political talks and a peace agreement.

"If we are to see real progress, decisive political action, which encompasses the whole of Sudan, is needed," Ban said in a report to the U.N. Security Council for the month of July.

The U.N.-AU force took over peacekeeping duties in Darfur in January from a beleaguered AU force. As of July 31, it had just over 8,100 military personnel and fewer than 1,900 police on the ground, out of a total of 26,000 that have been authorized.

Nigerian Gen. Martin Agwai, the commander of the force known as UNAMID, said in June that he expected the force to grow to 13,000 in three or four months, and he expressed optimism the force could reach its goal of 80 percent of the full deployment by year's end.

But that was before the most serious attack on UNAMID since it took over peacekeeping — a well-organized assault on a UNAMID patrol on July 8 that left seven peacekeepers dead and 22 wounded. As a result, the force heightened security and temporarily relocated staff.

Preparations for the deployment of additional troops and police "continue to be hampered by significant logistical challenges and insecurity," Ban said.

Increasing banditry in July also "substantially hindered UNAMID and humanitarian operations" as did aerial attacks and tribal clashes that erupted over disputed land rights in southern Darfur, he said.

The secretary-general stressed that even when fully deployed, "UNAMID cannot be a substitute for a political process."

The success of the new AU-U.N. chief mediator, Djibrill Yipene Bassole, "will hinge on the will of the parties to resolve their differences through dialogue" as well as international support for his efforts.

The U.N. and AU have tried for months to open new peace talks between Sudan and rebel groups following the failure of a 2005 agreement to stem the violence. Most rebel chiefs are boycotting the negotiations.

The violence erupted in early 2003 when ethnic African rebels took up arms against Sudan's Arab-dominated central government, accusing it of discrimination.