South Sudan's president has refused to pull back troops who seized a disputed border town from Sudan this week.

Addressing parliament Thursday, Salva Kiir said he will not order the withdrawal of southern forces from the town of Heglig, despite calls from the African Union and United Nations to do so.

He also said South Sudanese forces may enter another disputed border area, Abyei, if the United Nations does not force Sudan to withdraw from the area, which it seized last year.

Hostilities between Sudan and South Sudan this week have raised fears the countries will descend into an all-out war.

South Sudan says Sudanese warplanes dropped several bombs early Thursday in their first attack on a major southern town. South Sudan's Deputy Information Minister said five bombs dropped in the town of Bentiu were apparently aimed at a bridge.

Wednesday, Sudan announced it was pulling out of talks with South Sudan because of the south's takeover of Heglig, an oil-producing town.

The African Union is trying to mediate bitter disputes between the two Sudans stemming from the south's independence last year. But the talks in Ethiopia's capital have made little progress.

Key issues include borders, the sharing of oil revenue and the status of nationals in each other's territory.

Both countries have suggested the possibility of renewed conflict. Before their separation, north and south Sudan fought a 21-year civil war that ended with a 2005 peace agreement.

Sudan has accused South Sudan of launching attacks in Heglig Tuesday with the help of local rebels who have been fighting Khartoum since last June.

South Sudan says its forces were reacting to a Sudanese attack and entered Heglig in pursuit of Sudanese troops.

On Wednesday, the United States condemned military involvement on both sides and called on the two Sudans to withdraw all forces deployed across the border.

United Nations chief Ban Ki-moon urged South Sudanese President Kiir to consider holding a presidential summit to build confidence and assure people in Sudan and South Sudan that peace and dialogue are the only option for both sides.

Mr. Kiir and Sudanese President Omar al-Bashir were scheduled to meet last week, but the summit was cancelled because of an earlier round of fighting over Heglig.