Seven election workers were killed early Sunday in Niger's troubled western Tillaberi region when their vehicle hit a mine, the area's governor told AFP.
The blast also injured three of the workers, who included the heads of polling stations, Tidjani Ibrahim Katiella told AFP.
The accident came as Niger holds a presidential election runoff between frontrunner Mohamed Bazoum, loyal to outgoing president Mahamadou Issoufou, and former president Mahamane Ousmane.
The team had been sent by the Tillaberi branch of the National Independent Electoral Commission (CENI) to monitor the polling, the governor said.
The explosion occurred in the town of Dargol some 100 kilometres (60 miles) from Niamey in the so-called tri-border region where Niger, Mali and Burkina Faso converge.
The vast and unstable region has been under a state of emergency since 2017.
It was the scene of one of the worst civilian massacres suffered by the Sahel country in early January, when around 100 people were killed during an attack on two villages.
The twin attack occurred on the day election officials announced results from the first round of the presidential vote held on December 27.
Niger voted Sunday in a presidential run-off between two political heavyweights that is set to bring about the first democratic transition of power in the coup-prone country's history.
The world's poorest nation according to the UN's development rankings for 189 countries, Niger is also struggling with jihadist insurgencies that have spilled over from Mali and Nigeria.
Only 7.4 million of the country's 22 million people were eligible to vote on Sunday -- a reflection of the population's youthfulness.
Thousands of soldiers were deployed nationwide for the vote, set to usher in a peaceful handover between elected presidents, a first since Niger's independence from France in 1960.
Several foreign observer missions were also on hand including from the Economic Community of West African States (ECOWAS) and the Francophonie organisation of French-speaking countries.
Outgoing President Mahamadou Issoufou's decision to voluntarily step down after two five-year terms was welcomed in a region where many leaders have tried to cling to power.
"I'm proud to be the first democratically elected president in our history to be able to pass the baton to another democratically elected president," Issoufou said as he voted in the capital Niamey accompanied by his two wives.
His successor will be either his right-hand man Mohamed Bazoum, 60, or Mahamane Ousmane, who became the country's first democratically elected president in 1993, only to be toppled in a coup three years later.
Ousmane, 71, is running for president for the fifth time since his ouster in one of the four military coups the country has seen along with six elections.
Both candidates are stalwarts of Niger's political scene.
One voter, 29-year-old student Idrissa Gado, said the election is a "source of pride" for Nigeriens.
"The next president must act against the rebels, it's the great concern in Niger," Gado said as he voted in Niamey.
Whoever wins, 42-year-old tile-layer Ibrahim Kadi Mahmane said he hoped the new leader "doesn't forget the poor and the people living in villages".
- 'Fake voting cards' -
The CENI independent electoral commission said in a statement Sunday that "fake voting cards" had been in circulation in several regions of the country.
Voting in his native village of Zinder, Ousmane said: "If the citizens see that these elections were once again... rigged, I fear the situation will be hard to manage."
Political alliances loomed large, with Bazoum the strong favorite after winning 39.3 percent of the first-round vote on December 27.
"I hope that luck will be with the winner... I have many reasons to believe it's indeed on my side," Bazoum said as he cast his ballot.
The former interior minister has already sealed the support of the candidates who came third and fourth in the first round.
Ousmane, who took nearly 17 percent in the first round, can count on the support of a coalition of 18 opposition parties as well as Hama Amadou, previously thought to be the most formidable candidate against Bazoum.
Amadou was banned from running because of a conviction for baby trafficking which he has slammed as politically motivated.
The opposition filed an unsuccessful appeal against the result of the first round, claiming fraud, but has since given up its boycott of CENI.
- Vast challenges -
Bazoum campaigned on continuity with the previous government, which promised development while facing the world's highest birthrate -- an average of seven children per woman.
"To absorb this population growth would take incredible economic growth," a Western diplomat said.
An immense security challenge also awaits the victor.
A brutal jihadist insurgency is intensifying in the country's west, while Islamist militants from the Nigerian movement Boko Haram wage incessant attacks in the southeast.
Sunday's mine blast that killed seven election workers and injured three occurred some 100 kilometers (60 miles) west of Niamey in the so-called tri-border region where Niger, Mali and Burkina Faso converge.
The vast and unstable region, which has been under a state of emergency since 2017, was the scene of one of the worst civilian massacres in Niger's history in early January when around 100 people were killed during an attack on two villages.
The twin attack occurred on the day election officials announced results from the December 27 first round.