KIGALI, Rwanda — The United Nations World Food Program announced Thursday that it would not give any new contracts to three Somali businessmen who have been accused of diverting food aid to Islamist militants and that the agency would welcome an independent investigation into its Somalia operations.

For months, World Food Program officials had denied mounting allegations that the contractors that they use to haul hundreds of millions of dollars of food into Somalia were stealing some of the food and funneling it to Islamist militants trying to topple Somalia’s weak transitional government.

The decision follows a harsh new United Nations report, whose pointed findings were first disclosed in The New York Times this week, that said that up to half the food aid to Somalia may be getting diverted. The report also accused Somali officials of selling visas for diplomatic trips to pirates and militants.

The World Food Program is the single largest aid agency in Somalia and a lifeline to more than two million Somalis. On Thursday, the program’s executive director, Josette Sheeran, said, “The integrity of our organization is paramount, and we will be reviewing and investigating each and every issue raised by this report.” The program also said that some of the accusations in the new report “conflicted with operational facts and information.”

Somalia has lurched from crisis to crisis since 1991, when the central government collapsed. In the past two days, more than 50 people have been killed and 150 wounded in the Somali capital, Mogadishu, in vicious fighting between government forces and Al Shabab, the insurgent group that controls much of the country.

Witnesses said that the insurgents overran government positions on Wednesday, reaching within a mile of the presidential compound. African Union peacekeepers, who are helping to protect the embattled government, then pushed the insurgents back with tanks.

Heavy shelling continued to rock the capital on Thursday. Both sides, in dueling news conferences, claimed victory.

Somalia’s state minister of defense, Sheik Yusuf Mohamed Siad, known among fighters as White Eyes, said, “We have pushed the Shabab back and the government army is taking new positions at the front line.”

When asked if this fighting was part of an expected major government offensive, Sheik Yusuf said, “No, it’s not the planned government offensive, but soon these terrorist elements will be eliminated.”

Ali Mohamoud Raghe, a spokesman for Al Shabab, told reporters, “Our fighters launched an offensive against the government positions and reached victories.”

The fighting spawned a new exodus of refugees from Mogadishu’s center to the outskirts.

“I was shocked when I saw mortars hitting the neighborhood, house by house,” said Safiya Muhudiin, a mother of four. “I had to escape with my kids to a safe location.”