Uganda ordered internet service providers to block all social media platforms and messaging apps on Tuesday until further notice, a letter from the country's communications regulator seen by Reuters said.

Users had complained earlier on Tuesday that they were unable to access Facebook and WhatsApp, social media platforms being widely used for campaigning ahead of Thursday's presidential election in the East African country.

"Uganda Communications Commission hereby directs you to immediately suspend any access and use, direct or otherwise, of all social media platforms and online messaging applications over your network until further notice," said the letter from the commission's executive director to internet providers.

The commission's spokesperson Ibrahim Bbossa and government spokesperson Ofwono Opondo did not answer calls requesting comment. An aide to Minister of Information Judith Nabakooba said she was unable to comment at the moment.

A source in Uganda's telecom sector said the government had made clear to executives at telecoms companies that the social media ban was in retaliation for Facebook blocking some pro-government accounts.

The US social media giant said on Monday it had taken down a network in Uganda linked to the country's ministry of information for using fake and duplicate accounts to post ahead of this week's election.