Image copyright AFP Image caption Abdullah Abdullah is expected to face Ashraf Ghani in the run-off

The Afghan presidential election front-runner, Abdullah Abdullah, has won the backing of a key rival, forming the contest's first major coalition.

Zalmai Rassoul, the successor favoured by outgoing President Hamid Karzai, pledged his support for Mr Abdullah, the top candidate from the first round.

The second and final round, due in mid-June, pits Mr Abdullah against Ashraf Ghani, a former World Bank economist.

The vote comes as international forces prepare to leave at the end of 2014.

Millions of Afghans defied Taliban threats to take part in the first round of the election, held on 5 April.

Partial results have shown that none of the eight candidates won the 50% needed to claim outright victory, forcing the two top-ranked contenders - Mr Abdullah and Mr Ghani - into a second round.

Mr Rassoul, a former foreign minister, was no longer in the running for the presidency, having come third in the first round with just over 11% of the vote.

An influential power-broker, he is expected to boost Mr Abdullah's chances.

The announcement of a final official result from the first round is due on Wednesday. It has been held up by adjudication in fraud claims.