South Sudan is blaming Omar Bashir's Islamic regime in Khartoum for bombing a refugee camp in its country.

Four bombs fell in and around a camp called Yida in Unity State. No casualties were reported.

The United States called the attack abhorrent and outrageous and demanded that Sudan halt aerial bombardments immediately.

The camp is home to more than 20,000 people who have fled the fighting on the border between Sudan and South Sudan.

A team from Franklin Graham's Samaritan's Purse ministry has been to the camp, distributing food and other supplies.

Graham visited the camp just six days before it was attacked.

The largely Christian South Sudan became independent from the Islamic northern part of the country four months ago.

South Sudan President Salva Kiir said he believes Sudan is getting ready to invade his country.

A U.S. satellite monitoring group said Friday that Sudan's military is upgrading air bases near the border with South Sudan.