Two opposition candidates in South Korea's presidential election have agreed to forge an alliance.

Yoon Suk-yeol of the main opposition People Power Party and Ahn Cheol-soo of the minor centrist People's Party held a joint news conference on Thursday in Seoul.

Ahn, who ranks third in the polls, announced he would drop out of the race and back Yoon.

Ahn had first proposed merging their candidacies to Yoon in February. But he told reporters a week later that he hadn't gotten a response from Yoon and would continue his campaign.

Yoon and Lee Jae-myung of the ruling Democratic Party are neck-and-neck in the latest opinion polls. Observers believe Yoon's agreement with Ahn may affect the result of the election, which is just six days ahead.