Quote:NEW ORLEANS, Louisiana (CNN) -- New Orleans' Charity Hospital halted efforts to evacuate its patients after it came under sniper fire, according to Dr. Tyler Curiel, who witnessed the incidents.

The attack came as New Orleans Mayor Ray Nagin issued "a desperate SOS" for the thousands of people stranded in and around the city's convention center with no food or water and fading hope.

Curiel and his National Guard escorts, were returning to the hospital after dropping off patients at nearby Tulane Medical Center, when someone started shooting at their convoy of Humvees. (Watch the video report on the sniper's attack -- 1:06)

"We were coming in from a parking deck at Tulane Medical Center, and a guy in a white shirt started firing at us," Curiel said. "The National Guard (troops), wearing flak jackets, tried to get a bead on this guy. "

The incident happened around 11:30 a.m. (12:30 p.m. ET). About an hour later, another gunman opened fire at the back of Charity Hospital.

"We got back to Charity Hospital with with food from Tulane and we said, 'OK the snipers are behind us, let's move on,'" Curiel said. "We started loading patients (for transport) and 20 minutes later, shots rang out."

The National Guard soldiers told staff to get away from the windows, and evacuations were halted.

Charity Hospital has no electricity, no water and the only food available is couple of cans of vegetables and graham crackers.

Evacuations by boat were halted after armed looters threatened medics, and overturned one of their boats.

The sniper attacks were the latest incidents of violence that have disrupted efforts to help people in the flooded city.