A US Treasury official is urging East African governments to tighten the loopholes that allow illicit money from war-torn South Sudan to cross into regional capitals.

Under Secretary for Terrorism and Financial Intelligence Sigal Mandelker said in Uganda Monday that authorities in countries like neighboring Uganda must make it clear that "corrupt money is not wanted here."

It is widely believed that many South Sudanese government officials have invested heavily in real estate in cities such as Kampala, Uganda's capital, where it is possible to pay cash in real estate transactions.

South Sudan has had civil war since December 2013.

Advocacy groups say the international community should more strongly fight corruption in the oil sector that fuels South Sudan's civil war.

The US has imposed sanctions on some individuals.