Twenty years after Ghana's Kofi Annan stepped down as Secretary-General of the United Nations, Africa is once again in contention to lead the world's largest multilateral organization.

The continent has a single representative among the six declared candidates seeking to succeed António Guterres, whose second and final five-year term ends on December 31.

Senegal's former president, Macky Sall, is hoping to become the third African to occupy the UN's highest office.

The race entered a crucial phase on Thursday as the candidates took part in a public debate at the UN headquarters in New York, ahead of the Security Council's first informal straw poll scheduled for July 30.

If ultimately selected by the Security Council and approved by the UN General Assembly, Sall would become the first African Secretary-General since Kofi Annan left office in 2006, ending a two-decade absence of African leadership at the helm of the global body.