"Since September 3, when the influx into Ethiopia started, an estimated 25 000 refugees have found refuge in Ethiopia," said Adrian Edwards, spokesman for the UN High Commissioner for Refugees.

"With hostilities still ongoing in Sudan's Blue Nile State, we expect the numbers to continue rising," he added, pointing out that aerial bombings were reported on two days last week and on Monday.

Together with three other agencies, the UNHCR issued a joint appeal for $18.3 million to help an expected 35 000 refugees.

Many had crossed over to Ethiopia from the Kurmuk border point, and are sleeping in the open in villages close to the check point.

With bombings occurring close by, Edwards said the UNHCR and the Ethiopian government are working to move the refugees to safer locations.

About 3 000 people have so far been moved to the Sherkole camp, about 50km south-east, while the Ethiopian government is building two transit centres near the key border entry points to host newcomers.

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 this month, as the government moved to assert its authority within its new borders.