MANILA, Philippines ? American banking giant Citibank is investigating some "questionable" transactions initiated by a wealth management executive suspected to have swindled some clients of the foreign bank's Binondo branch.

"We have filed criminal and civil cases against him, supported by his own admissions and very strong documentary evidence. We are working closely with the relevant authorities who are assisting us in this matter," Citibank said in a press statement Friday.

The statement did not name the executive but banking sources said it was someone named Brian Ang, who was believed to have slipped out of the country as of posting time.

Citibank said the person has been terminated from his employment.

"Although the investigation is still ongoing, we believe this is an isolated case, and we are taking all necessary measures to protect our clients," the bank added.

The bank said it has identified a "small" number of affected clients.

"We are reviewing these accounts and our priority is to work closely with these clients to ensure a satisfactory outcome," the bank said.