Fresh aerial bombings in Sudan's border Blue Nile state are sending more refugees fleeing to Ethiopia, with 2,000 arriving in the last four days, the UN refugee agency said on Friday.

"Aerial bombings in Sudan's Blue Nile state are driving a new wave of refugees into Ethiopia," said the UN High Commissioner for Refugees in a statement.

"In the last four days, nearly 2,000 Sudanese refugees have arrived in western Ethiopia amid tightening security at the border area of Kurmuk," said the UNHCR, noting that most of them are women, children and the elderly.

Since fighting first erupted in September, a total of 28,700 people have fled the border state, and the UNHCR expects more refugees to arrive in Ethiopia.

Khartoum is engaged in military operations against rebel movements in three separate regions along Sudan's volatile border with the south, which gained full independence on July 9.

The conflict in South Kordofan state erupted just one month before southern secession, between the Sudanese army and Nuba militiamen who fought with the SPLA, the former rebel army of the south, during their decades-long war with the north.

The fighting spilled into nearby Blue Nile state at the beginning of September, as the government moved to assert its authority within its new borders.