GENEVA - The U.N. refugee agency (UNHCR) is making plans for hundreds of thousands of people spilling over Sudan's borders to escape violence, officials said on Tuesday, many of them forced to places they have already fled from.

UNHCR officials told a Geneva briefing they are poised for 270,000 people to flee over Sudan's borders - a preliminary planning figure that includes Sudanese refugees crossing into South Sudan and Chad as well as South Sudanese returning home.

The estimate so far only covers two of Sudan's seven neighbours as projections for Egypt, Eritrea, Ethiopia, Central African Republic and Libya are not yet ready.

Sudan hosts more than 1 million refugees, many of whom have fled conflicts in neighbouring countries such as South Sudan. In addition, 3.7 million of its own people are internally displaced, U.N. data shows.

The UNHCR representative in South Sudan, Marie-Hélène Verney, said the agency was planning for 125,000 South Sudanese refugees temporarily residing in Sudan to return home and for some 45,000 new Sudanese refugees to cross into South Sudan.

"We know the returns will take place first. They are mostly from Khartoum and the refugee flows are likely to be afterwards," she said. South Sudan officials say 10,000 refugees have arrived there in recent days.

UNHCR said it expected many to return to parts of South Sudan that are "extremely fragile as a result of conflict, climate change or food insecurity - or a combination of all three".

UNHCR Chad representative Laura lo Castro, who was at the border last week, said they were planning for 100,000 refugees from Sudan in a worst-case scenario. Some 20,000 have already arrived, she added.

"It's really a run against the clock because the people are really very much in need," she said by video link from N'Djamena. UNHCR added that it had reports of people starting to arrive in Egypt, but no exact numbers.

(Reporting by Emma Farge; Editing by Kevin Liffey)