More than 136 000 people have fled Sudan's southeastern Sennar state since the paramilitary Rapid Support Forces began a series of attacks on towns, the United Nations said on Wednesday, the latest wave of displacement caused by Sudan's almost 15-month long war.

They join nearly 10-million people driven from their homes since war broke out between the RSF and the regular army. The war has sparked accusations of "ethnic cleansing" and warnings of famine, mainly in RSF-controlled areas across the country.

The RSF on June 24 began a campaign to seize the city of Sennar, a trading hub, but quickly turned to the smaller towns of Sinjah and al-Dinder, prompting an exodus of civilians from all three, mainly to neighbouring al-Gedaref and Blue Nile states.

Images on social media showed people of all ages wading across the Blue Nile.

Activists in both states say there is little shelter or food aid for the incomers. In Gedaref, they faced an onslaught of heavy rain while stranded in the state capital's main market with no tents or blankets after schools that had served as displacement centers were emptied by the government, the local resistance committee said.

The UN's International Organization for Migration said in a statement that since June 24, an estimated total of 136 130 people had been displaced in Sennar.

The state was already home to more than 285 000 people displaced from Khartoum and al-Gezira states, meaning that many of those leaving over the last two weeks were likely to have been displaced for the second or third time. It also said that villages in Gedaref state, one of several possible targets for the RSF campaign, had also seen an exodus.

To the west of the country, local activists said at least 12 people were killed by artillery fire on a livestock market on Wednesday in the city of al-Fashir which has seen a months-long fight for control and an exodus towards nearby towns and displacement camps.