"(They) were abducted this morning," said the source, who gave no details about the number or nationality of the kidnap victims.
"The kidnappers are probably Nigerian pirates," the source said. "Cameroonian forces have launched search operations."
An official at Douala port confirmed the kidnappings, and said the seamen had been taken after their ship came under attack.
"We don't know the number of abducted seamen for the time being," the official said.
The Gulf of Guinea, whose coastline stretches in a huge arc from Liberia to Gabon, is notorious for piracy as well as oil theft, illegal fishing and human and drugs trafficking.
According to the International Maritime Bureau (IMB) 62 seafarers were taken hostage or abducted in the area in the first half of 2019.
The Gulf of Guinea accounted for 73 percent of kidnappings and 92 percent of hostage-takings at sea worldwide, it says.
The 17 countries in the Gulf of Guinea and adjacent regions have limited surveillance and maritime defence capabilities.
They have been trying for several years to bolster their means of intervention and to put in place closer collaboration.