ES SALLAM, Sudan — Britain and Germany called yesterday for tougher action against Sudan to end four years of bloodshed in Darfur, where the new U.N. humanitarian chief warned of a possible collapse in the massive effort to aid refugees from the violence.

Britain’s Prime Minister Blair called the actions of Sudan’s government “unacceptable” and Germany’s Chancellor Merkel called the Darfur region’s suffering “unbearable.”

Experts say the few economic sanctions imposed on Sudan by America and the European Union have had little effect, and Secretary-General Ban recently voiced his frustration at Sudan’s government for refusing to let a 22,000-strong U.N. peacekeeping force into Darfur.

On his first tour of the region, John Holmes, the new U.N. undersecretary-general for humanitarian affairs, said the aid effort there — the largest in the world with some 14,000 aid workers and over $1 billion spent — was “fragile.”

“If the situation deteriorates, it could collapse,” Mr. Holmes said.

Sudan has angered the West by repeatedly rejecting the U.N. peacekeepers. And prosecutors at the International Criminal Court in the Hague, Netherlands, linked Sudan’s government to atrocities in Darfur, naming a junior minister as a war-crimes suspect.

“The actions of the Sudanese government are completely unacceptable,” Mr. Blair said at E.U. anniversary meetings in Berlin. “We need to get a new resolution in the United Nations, which extends the sanctions regime … We need to consider, in my view, a no-fly zone.”

Protected by its top ally, China, which has veto power at the Security Council, Sudan has so far avoided any harsh U.N. sanctions for the atrocities committed in Darfur.

Some 4 million people are in need of aid, either living in refugee camps or threatened by the fighting between local rebels, the Sudanese government and the progovernment janjaweed militia, the United Nations says.

The conflict has chased 86,000 more people from their homes this year alone, the United Nations says, blaming the vast majority of these new refugees on violence perpetrated by Sudanese government forces and the janjaweed.

Overall, more than 200,000 people have been killed and more than 2 million others — mostly ethnic African villagers — have fled to refugee camps, where most survive on aid provided by U.N. agencies or private groups.