Oscar-winning actor turned activist Sean Penn, who has been working on Haiti aid after the massive January 12 earthquake, urged US lawmakers Wednesday to help reopen Haitian hospitals that have shut down for lack of funds.

"The hospitals that do exist have got to be staffed, supplied, administrated to," said the actor, who set up the J/P Haiti Relief Organization after the disaster. He was at a refugee camp in Port-au-Prince for four months, and is headed back later this month.

"In this city of ruins, five fully functional hospitals have been allowed to close despite these emergent disasters due to financial under-support and the inertia of 'protective' over-scrutiny," Penn argued before the Senate foreign affairs committee.

He was joined in the push for more aid funding by USAID coordinator Christopher Milligan who said the United States role should include helping "rebuild public health care" in Haiti.

The Caribbean nation was already the poorest nation in the Americas before the earthquake shattered much of its capital.

January's quake left more than 1.3 million people in need of shelter and claimed as many as 300,000 lives.

Penn said the first cases of diphtheria had been reported on the golf course in Petionville where some 50,000 people are living out in the open just as the hurricane season churns to a start.

The US actor said the vaccination shortage was critical.

"There is no great immunization campaign, this is simply not true," though one is needed, he said.

Penn stressed that "the Haitian people are as strong and resilient as any I have ever seen. There are great lessons of character for our country to learn from Haitians."

Yet given the crisis on the ground he warned: "we must also not underestimate the likelihood of violent social unrest."

"As Americans, we should call on the governmeent of Haiti and on our own government to acknowledge that a state of emergency still exists. To demand full transparency in the way that aid is distributed and accountability for how aid organizations advertise themselves in the solicitation of funds," Penn urged.

Some 100 countries have pledged about 15 billion dollars in aid to Haiti. The United States is gearing up to vote on its aid of about 3.5 billion dollars.