Washington ? President Barack Obama, declaring the violence in Darfur and inaction in the face of its worsening humanitarian crisis are “not acceptable,” pledged Tuesday to work more closely with the United Nations to bring peace to western Sudan’s conflict-wracked region.

Obama’s response during a nearly hourlong Oval Office meeting with U.N. Secretary-General Ban Ki-moon significantly ratcheted up the U.S. response to Sudan President Omar al-Bashir’s expulsion of 13 aid groups.

Obama described Darfur as an ongoing crisis that has only gotten worse with the Sudanese president’s decision to kick out some of the most important nongovernmental aid organizations after the Hague-based International Criminal Court issued an arrest warrant charging him with war crimes and crimes against humanity in Darfur.

The U.N. estimates that the decision threatens more than 3 million people with the loss of food aid, health care or suitable drinking water.

“It is not acceptable to put that many people’s lives at risk,” Obama said. “We need to be able to get those humanitarian organizations back on the ground.”

Ban, in turn, told Obama that 2009 is a “make-or-break” year for the organization and its member countries, and that he hopes the United States will work with the international organization to address global warming and the humanitarian crisis in Darfur.

Ban’s top priority this year is to encourage global leaders to adopt a new international climate treaty at a conference in December in Copenhagen.