Officials say suspected Sudanese warplanes have bombed a refugee camp in neighboring South Sudan, causing injuries and deaths.

A local journalist tells VOA that planes bombed the Yida refugee camp in South Sudan's Unity state Thursday afternoon. He says up to 20 people were killed, with many others wounded.

There has been no independent confirmation of the casualty figures, and the motive for the attack was not clear.

Late Wednesday, the United States strongly condemned a separate airstrike by Sudan on its southern neighbor.

South Sudan says Sudanese jet fighters bombed an area of Upper Nile state on Monday, killing seven people.

The U.S. State Department called the attack provocative, and said such bombardments increase the chance of war between Sudan and South Sudan.

The two countries have been at sharp odds since the south split from Sudan in July, six years after the end of Sudan's long north-south civil war.

Sudan has accused the south of supporting rebels in two states along the border, Southern Kordofan and Blue Nile. Yida, which sits about 40 kilometers from the border, hosts thousands of refugees from Southern Kordofan.

Speaking to reporters Thursday, South Sudan President Salva Kiir denied that his country supports the rebels and suggested Sudanese President Omar al-Bashir is planning to retake South Sudan.

Sudan has recognized the south's independence but the sides have yet to resolve issues over borders — especially in the oil-rich Abyei region — and the sharing of oil revenue. The south took over most of Sudan's oil in the split but pipelines to the sea run through the north.