Diphtheria is an infectious disease that chiefly affects the throat and upper airways and is spread through physical or respiratory contact. It is fatal in around 5 to 10 percent of cases, according to the World Health Organisation.

Last seen in Venezuela in 1992, diphtheria has been spreading in the southern jungle state of Bolivar, according to a statement by two local public health associations on Thursday.

"We're very worried because there could be an epidemic in the rest of the country," infectious disease specialist Doctor Ana Carvajal, one of the authors of the report, told Reuters.

The statement said 17 people had died from diphtheria in Venezuela, citing local media. Opposition lawmaker and oncologist Jose Manuel Olivares earlier this week put the number at 22.

Reuters could not independently confirm the reports.

The leftist government of Nicolas Maduro, which has stopped publishing weekly health statistics, has not spoken about the alleged outbreak. The Health Ministry did not immediately respond to a request for comment.

Venezuela's economy is in a tailspin, with shortages of items from disinfectant to chemotherapy drugs crippling the health sector and leaving 30 million Venezuelans struggling to access basic medical care.

The ruling party governor of Bolivar said this week that there was no proven case of diphtheria but that the state was already vaccinating people.

"400,000 doses of vaccines have arrived for Bolivar to guarantee all citizens can be vaccinated," governor Francisco Rangel said on Twitter, criticising those seeking to spread "panic." He tweeted pictures of people being vaccinated.

Venezuela's opposition, which is pushing to remove the unpopular Maduro in a recall referendum this year, has slammed what it says is the government's attempt to hide the outbreak.

"The resurgence of diphtheria shows the failure of health policies," lawmaker Olivares said.

Diphtheria was once a major cause of illness and death of children but is now very rare in developed countries due to vaccinations.