It also warned its NGO partners that the bloc would cut funding if they breached ethical standards, underscoring its "zero tolerance" approach towards misconduct.
"We expect Oxfam to fully clarify the allegations with maximum transparency as a matter of urgency, and we're ready to review and, if needed, cease funding to any partner who is not living up to the required high ethical standards," spokeswoman Maja Kocijancic told reporters.
Read more: Peacekeepers commonly barter goods for sex, says UN study
Oxfam, which received €1.7 million ($2.1 million) in funding from the EU for work in Haiti in 2011, is being accused of covering up the episode and failing to warn other aid agencies about the employees involved, which allowed them to get jobs among vulnerable people in other disaster areas.
Oxfam Deputy Chief Executive Penny Lawrence resigned from her position on Monday, taking " full responsibility" for the scandal.
"Over the last few days we have become aware that concerns were raised about the behavior of staff in Chad as well as Haiti that we failed to adequately act upon," Lawrence said in a statement.
"It is now clear that these allegations—involving the use of prostitutes and which related to behavior of both the country director and members of his team in Chad—were raised before he moved to Haiti," she added.
Oxfam has denied accusations of a cover-up, saying it launched an internal investigation the moment it became aware of the allegations.
The probe led to the sacking of four Oxfam employees, the charity said on Friday, adding that three people, including the country director, resigned before the end of the investigation.
Read more: Haiti, the UN and the unintended effects of a peace mission
British government threatens to cut fundingBritish Prime Minister Theresa May's spokesman told reporters that Development Secretary Penny Mordaunt was meeting Oxfam representatives on Monday.
"DFID (the Department for International Development) have taken action in this area to try and further strengthen systems to enforce a zero tolerance approach. There are a number of measures they have taken but we want to go further in this area," he said.
DFID gave Oxfam nearly £32 million (€36 million) last year.
The British government has also threatened to cut off funding to Oxfam or any other aid agency that doesn't cooperate with the government's efforts to rein in misconduct.
Oxfam, which has announced new measures to tackle sexual abuse cases, said it would share with authorities all the information it had relating to the scandal in Haiti.
ap, law/kms (AFP, AFP)