PRISTINA (Reuters) - Three opposition parties in Kosovo said on Tuesday they had united and planned to form a government, trying to undercut Prime Minister Hashim Thaci whose ruling PDK party won an election but was left seeking partners.

The second-placed Democratic League of Kosovo (LDK) said it had reached a coalition deal with the Alliance for the Future of Kosovo (AAK) of former guerrilla commander Ramush Haradinaj and a small party led by defectors from Thaci's Democratic Party of Kosovo (PDK).

Under the constitution, Kosovo's president will first give Thaci the mandate to form a government as leader of the winning party in Sunday's election. He will struggle, however, without the support of at least one of the parties that on Tuesday said they would unite to deny him a third four-year term.