The Philippines said Thursday that some of the 14 children who died after receiving a controversial vaccine showed signs of "severe dengue", as investigators probe the drug whose use was suspended due to health concerns.

More than 830,000 Filipino schoolchildren were injected with Sanofi's Dengvaxia vaccine last year in the world's first public dengue immunization program.

But the country stopped the sale and distribution of Dengvaxia last month after Sanofi warned the vaccine could worsen symptoms for people who had not previously been infected with the virus.

Philippine authorities are also pursuing criminal and public health safety investigations into any links between the drug and the deaths of 14 schoolchildren who died months after being vaccinated.

"The findings of the forensic pathologists are consistent with severe dengue or dengue shock syndrome," Health Secretary Francisco Duque told ABS-CBN News, referring to the results of autopsies on some of the deceased children.

Duque also said he had asked Sanofi to refund around P1.5 billion worth of unused vaccines.

Officials at the government's Public Attorney's Office (PAO), which has been conducting autopsies on some of the dead children, have been trying to build a criminal case by proving the deaths were linked to the vaccine.

• READ: PAO sees 'pattern' in deaths of 4 Dengvaxia recipients

The health department has also commissioned independent experts to pursue a separate inquiry in the interest of public health and safety, though the results of this study have not been disclosed.

"They may or may not be investigating the same cases or deaths and would be using different methods of investigation," Health Undersecretary Eric Tayag told AFP.

Sanofi said in a statement on Thursday that it was "saddened" to learn of the deaths of the children.

"Up to this date, there has been no death established to have been causally linked to the dengue vaccine, not even among the 40,000 people involved in the clinical trials conducted across 15 countries," the statement added.

In November last year, the French company released findings of a new study that showed Dengvaxia could lead to severe infections for vaccinated people who caught dengue for the first time.

The disclosure triggered a public furor, with some parents blaming the vaccine for their children's deaths and a number of legislators accusing the government of endangering public health.

Dengue, a mosquito-borne illness, is a leading cause of serious illness and death among children in some Asian and Latin American countries, according to the World Health Organization.

• READ: SC seeks DOH comment on Dengvaxia risks