Liberia's new government needs massive foreign aid to cement peace and rebuild a nation ruined by more than a decade of civil war, but must revive its economy to avoid becoming dependent on donors, a top U.N. official said. "Economic hope I think is crucial. There is a window of opportunity here, but that would not last forever," Alan Doss, head of the United Nations mission in Liberia, UNMIL, told Reuters in an interview late on Thursday. Former World Bank economist Ellen Johnson-Sirleaf takes over on Monday as Africa's first elected female head of state after winning polls last year aimed at ending years of instability. "Considerable assistance from donors, including debt relief, will be needed in the early years. That is inevitable because there are things that government needs to do: infrastructure has to be built, and so on," Doss said. "Right now, Liberia does not have the means to do that itself. It will need external assistance, a lot of aid. I do not think aid or public sector aid in the long term has the answer. I think it has to be private sector investment-led," he said.