Voting in Niger's presidential and legislative elections stretched into a second day on Monday in areas where logistical problems prevented polling the previous day, delaying the preliminary election results.

President Mahamadou Issoufou, who has vowed to crush Islamist militants and reduce the country's deep poverty, is running against 14 other candidates, including Seyni Oumaru, leader of an opposition coalition.

Critics say Issoufou is repressive and arrested opposition supporters in the run-up to the vote. The government says such criticism is politically motivated.

Polls will be open in four of the eight regions in the landlocked Saharan country: the northeastern Tahoua region, and Zinder, Diffa and Tillaberi, in the east, southeast and west respectively, observers said.

"The vote restarted on Monday in areas where the polling stations didn't work yesterday," said Kadi Moustapha, a spokesperson for the West African Network for Edification and Peace observer group.

Diffa is under a state of emergency after Islamist militant group Boko Haram, based in neighbouring Nigeria, staged a series of attacks there in recent months.

Niger produces uranium and oil but is desperately poor and has one of the world's highest fertility rates.