Almonor said around 20 houses in the area were also set ablaze, but that no details were yet available on possible victim numbers inside the homes.
Nearby Justinien University Hospital was overwhelmed with patients as the injured were transported to the facility.
"We don't have the ability to treat the number of seriously burned people," a nurse told AFP. "I'm afraid we won't be able to save them all," she said.
A doctor at the hospital told local radio station Magik9 that two people had died there and that 40 other patients were seriously injured.
"The people are burned on more than 60 percent of their body," she said.
Haitian prime minister Ariel Henry decreed a period of national mourning following the explosion he said had left dozens of people injured.
"I learned with sadness and emotion the terrible news of the explosion of a gas tanker last night in Cap-Haitien," Henry tweeted.
"Three days of national mourning will be decreed throughout the land, in memory of the victims of this tragedy which has devastated the whole Haitian nation." Henry promised field hospitals would be rapidly deployed to help care for the blast victims.
The Caribbean nation has never produced enough electricity to meet the needs of the whole population. Even in well-off parts of the capital, the state-run Haiti electric utility only provides, at most, a few hours of power a day.
Those who can afford it rely on pricey generators, which are no help in the face of the severe fuel shortage caused by gangs blocking access to the country's oil terminals in the capital and its outskirts.
In recent months more than a dozen vehicles transporting fuel have been attacked by gangs demanding ransoms for the drivers' release.
Demonstrators took the streets as recently as Monday protesting the resulting rise in gasoline prices.
The lack of fuel is also damming up water access, in a country where many people rely on private companies to deliver water by truck to at-home systems.
And with no guarantee of steady power or water supply, health care providers have been forced to drastically cut back their services.
Chronically-unstable Haiti was also plunged into a new political crisis with the assassination of the late president, Jovenel Moise, in July.
Four senior Haitian law enforcement officials have been detained and several dozen arrested in connection with the investigation.
But five months after Moise's assassination, doubts remain over who ordered the attack, during which not a single police officer guarding the president was injured.