Sudanese government mechanized infantry columns supported by helicopter gunships launched a massive local offensive in three southern provinces, forcing hundreds of thousands of civilians from their homes. 

The Sudan People's Liberation Army and Christian Solidarity International reported that Khartoum's forces attacked rebel-held villages in Bahr el-Ghazal and Western and Eastern Upper Nile provinces on 20 April. Heavy fighting also reported on the road between the towns of Wau and Gogrial, 620 miles southwest of Khartoum. 

Apparently, it took a few weeks for Khartoum to move all their pieces into position.
Sudan People's Liberation Movement spokesman Samson Kwaje told the press on 15 April that the offensive that started in late March had intensified and that "the government has launched a massive military offensive in Western Upper Nile and Bahr el Ghazal region". - Adam Geibel