The U.N. agency for Palestinian refugees warned Thursday that 11 typhoid cases among civilians from a besieged Palestinian refugee camp on the outskirts of the Syrian capital may be just "the tip of the iceberg."

Medical teams from the U.N. Relief and Works Agency in Yalda, an area east of the Yarmouk camp hosting displaced Palestinian refugees and Syrian civilians, confirmed six typhoid cases on Tuesday and five cases on Wednesday.

UNRWA said there were "credible reports" of a typhoid outbreak in the region with suspected cases in Yarmouk, Yalda and two other areas, Babila and Beit Sahem.

UNRWA spokesman Christopher Gunness said the agency has not had access to Yarmouk since March 28 and has not gotten to other adjacent areas except Yalda.

"We fear that the cases we are seeing are the tip of the iceberg, which is a source of grave concern, given that in contexts such as Yarmouk, typhoid can be a killer," Gunness told The Associated Press. "We cannot allow this contagion to go unaddressed."

According to the U.S. Centers for Disease Control and Prevention, typhoid is caused by the bacteria Salmonella typhi which is spread by eating contaminated food or drinking contaminated water. It is usually treated with antibiotics, but can be fatal in some cases without treatment.

UNRWA Commissioner-General Pierre Krahenbuhl said in June that the Palestinian refugee population in Yarmouk dropped from 160,000 when the Syrian war began in 2011 to 18,000 before the Islamic State extremist group entered the camp in early April—and several thousand have fled since then.

Gunness again urged full humanitarian access to Yarmouk "so that we can treat its embattled civilians, now threatened with a killer disease."

"Who knows how many women, children, the elderly, the extremely vulnerable may be at death's door," he said.