The British High Commission in Abuja said it was monitoring the situation closely and urged authorities to allow Independent National Electoral Commission (INEC) staff “to do their job in safety, without intimidation”.
“Extremely concerned by reports, including from @UKinnigeria observers, of military interference in the election process in Rivers State,” the commission said on Twitter.
Counting is on going after elections on Saturday for governors in 29 of Nigeria’s 36 states, all state assemblies and administrative councils in the Federal Capital Territory of Abuja and the INEC is expected to announce the results in the coming days.
It is the second time in a fortnight that the country has gone to the polls after presidential elections in February that saw President Muhammadu Buhari clinch a second term in office.
While voting was largely peaceful, incidents of violence marred the poll, with reports of abductions and killings as well as concerns over vote buying and a strong military presence.
Police on Sunday said three INEC staff were kidnapped in northern Katsina state in an ambush that left a policeman dead during voting on Saturday.
And on Saturday authorities said a local election observer in southeastern Enugu state was killed by a stray bullet as police attempted to disperse demonstrators.
Regional elections are fiercely contested in Nigeria, where governors are powerful and influential figures, controlling state finances and responsible for key areas from education to health.
Most domestic and international observers said last month’s presidential vote was credible, despite well-documented problems.
But tensions remain high as the beaten Peoples Democratic Party (PDP) challenges the result in court.
The PDP is hoping for victory in some of the 22 states currently run by Buhari’s ruling All Progressives Congress (APC).
Adding to tensions in Rivers — a volatile state in the oil producing Delta region — is a court ruling barring any APC candidates from standing in the gubernatorial election because of procedural irregularities in the selection process.
Incumbent PDP governor Nyesom Wike, who is considered a favourite to be reelected, has accused the military of complicity in the killing of 16 people in the Abonnema area of the state.
Three people were killed in Rivers on Friday, according to the transport minister, following clashes between APC and PDP supporters.