Nearly 600 refugees left Tanzania on Thursday to return to their homes in Burundi, the first batch in a mass repatriation agreement between the two countries, the United Nations said.

According to the Human Rights Watch, more than 400,000 Burundians fled their country following the violence that hit the country in 2015. They headed to neighbouring countries including Tanzania, Uganda, Rwanda and the Democratic Republic of Congo.

Tanzania and Burundi signed the agreement on August 24 to facilitate the return of the refugees, a move Bujumbura says demonstrates a return to peace, calm, confidence, and the stability of the country.

Tanzania hosts some 183,000 Burundian refugees.

The U.N. refugee agency, UNHCR, said 590 Burundian refugees had returned on flights that it had organised with the U.N.'s International Organization of Migration (IOM).

The Burundi government has often said the security situation in the country is back under control following incidents of violence that arose ahead of the presidential election in 2015.

The violence was sparked by a declaration by President Pierre Nkurunziza that he would run for a third term, one which he went on to win in the July poll.

The country's opposition accused President Nkurunziza of going against the constitution by running for a third term, then rallied their supporters to the streets.

The repatriation of refugees comes ahead of a presidential election in Burundi scheduled for 2020, which the government has pledged will be free and fair.

While addressing the United Nations General Assembly last month, Burundi's Minister of Foreign Affairs Ezechiel Nibigira said there is an increasingly favourable climate for free, transparent and calm election.