Sudanese rebels have fired mortar bombs into the capital of South Kordofan state where they rose up against the government two years ago, the insurgents said on Wednesday.
The shelling of Kadugli was in retaliation for an air strike last Sunday that killed two children, Arnu Ngutulu Lodi, spokesman for the Sudan People's Liberation Army-North (SPLA-N), said.
"We targeted military positions" with five mortar rounds, he told AFP.
It was not immediately clear whether there were any casualties.
Government troops were responding on Wednesday with outbound fire against rebel positions, Lodi said.
Two children, aged 10 and seven, were killed when a government Sukhoi jet bombed Tanasa village, near Buram town southeast of Kadugli on Sunday, according to the rebels.
International human rights groups have accused the Sudanese military of "indiscriminate" aerial attacks, while the government several months ago said the rebels killed civilians in previous shelling of Kadugli.
The ethnic SPLA-N belongs to an alliance of Sudanese insurgents aiming to topple the Khartoum regime and install a government more representative of the country's diversity.
The military says it is beginning an operation to crush the insurgents.