UN investigators took samples of foul-smelling waste trickling behind a Nepalese peacekeeping base toward an infected river system Wednesday, following accusations that excrement from the new unit caused the cholera epidemic that has sickened more than 4,000 people in the earthquake-ravaged country.

Associated Press journalists who were visiting the base unannounced happened upon the investigators.

Mission spokesman Vincenzo Pugliese confirmed that the military team was testing for cholera — the first public acknowledgment that the 12,000-member force is investigating allegations that its base played a role in the outbreak.

Meanwhile, the epidemic continued to spread, with cases confirmed in two new departments in Haiti's north and northeast, said UN Office for the Co-ordination of Humanitarian Affairs spokeswoman Imogen Wall.

At least 303 people have died and 4,722 have been taken to hospital.

International aid workers and the United Nations are focusing their efforts on stemming the spread of the outbreak, first noted on Oct. 20. But Haitians are increasingly turning their attention to its origins: How did a disease which has not been seen in Haiti since the early 20th century suddenly erupt in the countryside?

UN peacekeepers from Guatemala take a sample of excrement next to the Nepali UN base in Mirebalais, Haiti, on Wednesday. (J.Katz/Associated Press)The mission strongly denies its base was a cause of the infection.

Pugliese said civilian engineers collected samples from the base last Friday that tested negative for cholera and the mission's military force commander ordered the additional tests to confirm. He said no members of the Nepalese battalion, whose current members arrived in early October for a six-month rotation, have the disease.

The unit's commander declined to comment.

River infected

Local politicians, including a powerful senator and the mayor of Mirebalais, are pointing the finger at the Nepalese peacekeeping base, which is perched above a source of the Meille River, a tributary to the Artibonite River on Haiti's central plateau.

The Artibonite River has been the source of most infections, which remain concentrated in the rural area surrounding it — mostly down river from the mouth of the Meille.

"They are located exactly where the sickness started," said Mirebalais Mayor Laguerre Lochard, who is also running for Senate.

Area residents are also blaming the base. A young man walked by its gate laughing and chanting, "Co-co-cholera. Cholera MINUSTAH" — referring to the peacekeeping mission by its French initials.

Cholera is almost unheard of in the Western Hemisphere. It is endemic to Nepal, which suffered outbreaks this summer. A recent article in the Japanese Journal of Infectious Diseases about outbreaks in 2008-09 said the strain found by researchers was Vibrio cholerae O1 Ogawa biotype El Tor.

That is the same strain that has been identified in Haiti, said epidemiologist Eric Mintz of the U.S. Centers for Disease Control and Prevention. But he cautioned that the strain is common and the description too general to be a "smoking gun" that would identify the strain's country of origin.