The attack hit a convoy carrying a government official responsible for land ownership issues in part of the Southwest, who was killed.
The area’s prefect survived the attack but was injured, the activist said.
The report was confirmed by sources in the security services, but the government in Yaounde would neither confirm nor deny it.
Unrest blamed on armed separatists has increased in recent weeks in the Northwest and the Southwest, two English-speaking territories that became part of the mainly French-speaking country after independence in 1960.
The attack took place ahead of elections to the Senate which are to be held on Sunday, with authorities banning certain types of vehicle on roads in the Southwest and urging people to “remain calm and vigilant”.
Senate elections do not take place by universal suffrage in Cameroon. Municipal councillors across the country vote for 70 members of the upper house, while appointees to the 30 remaining seats are named by President Paul Biya.
On Tuesday, a Tunisian was killed in the Southwest during an operation aimed at freeing four hostages — two Tunisians and two Cameroonians — who were kidnapped by an armed group on March 15, state television reported.
The government said the Tunisian was “killed by his abductors” and that “four terrorists” also died in the operation.
A deputy district administrator and a regional employee of the social affairs ministry were both kidnapped in the Northwest in February. Both are still missing.
Hardline anglophone separatists committed to winning independence for their self-proclaimed republic of “Ambazonia” have described security and defence personnel as “occupation forces”.
In the past three months, the conflict between separatists and the army has become increasingly serious, as separatists urge the armed forces and government officials to leave the territory.
New separatist groups are emerging, while the Yaounde authorities have reduced their communcations about the conflict to a minimum.