Former Foreign Minister Abdullah Abdullah received a boost in the race for the Afghan presidency on Sunday when one of the pre-election favorites dropped out and backed his team ahead of next month's expected run-off.

Zalmay Rassoul, who finished third in April's first round with 11.5 percent of the vote, told journalists in Kabul he had endorsed Abdullah to strengthen national unity, and because the pair campaigned on similar platforms.

Preliminary results showed Abdullah and his closest rival, former World Bank economist Ashraf Ghani, sharing over 75 percent of the vote, but neither winning an absolute majority.