Voting has been extended in Zambia in areas where heavy rainy prevented polling stations from opening in Tuesday's presidential elections.

"Extreme thunderstorms" grounded flights from taking material to remote parts, officials are quoted as saying.

Polling stations that opened late because of the weather stayed open beyond the 16:00 GMT closing time.

The vote was caused by the death of Michael Sata last year and the winner will serve his remaining term.

It is expected to be a close contest between Edgar Lungu from the governing Patriotic Front and Hakainde Hichilema from the United Party for National Development.

The BBC's Nomsa Maseko in the capital, Lusaka, says candidates have vowed to improve the education system and create jobs.

'No control over the weather'

Our reporter says most polling stations are not easily accessible by road in rural areas - and there were heavy downpours across the country on Tuesday.

"We have no control over the weather," the AFP news agency quoted elections director Priscilla Isaacs as saying.

The electoral commission said in one area where voting material was unable to be delivered on Tuesday, it will have to be first transported by boat, then polling officers will have to walk for three hours before taking ox-drawn carts to polling stations, the agency reports.

Counting of the ballots that were cast in most areas has already begun and results are due to be announced within 48 hours.

Michael Sata, who was Zambia's fifth president, died in London in October at the age of 77.

Zambia is due to hold its next general election in 2016.