Ban said the charges filed against Ashe "go to the heart of the integrity of the United Nations," according to a statement from his spokesman Stephane Dujarric.
Ashe, who served in the top UN post for a year from September 2013, was charged with taking USD 1.3 million in bribes from Chinese businessmen, partly to push for a real estate project in Macau.
In exchange for payments from Ng, Ashe submitted a request to UN Secretary-General Ban Ki-moon "which claimed that there was a purported need to build the UN Macau Conference Center."
The spokesman said UN officials first learned of the charges when media reports surfaced today and had not been contacted by the US attorney's office to help in the investigation.
Dujarric told reporters that UN officials had not been able to find the letter mentioned in the complaint, which was not listed in Ban's log of official correspondence.
"We've just learned of these very serious allegations this morning," said Dujarric.
The United Nations scheduled a news conference for 3 pm (1900 GMT) by the current president of the General Assembly, Mogens Lykketoft of Denmark.
