"JD(U) state executive has taken a decision to fight the bypoll for 10 Assembly seats next month together with RJD and Congress," he told reporters.
"The party has authorised state President Basistha Narayan Singh to initiate talks with parties opposed to BJP to unitedly fight the bypoll in August end," Kumar, former Chief Minister of Bihar said.
Kumar, however, said he is yet to hold talks with RJD President Lalu Prasad in this regard.
Bihar JD(U) President Basistha Narayan Singh separately told reporters about the decision to fight the bypoll with RJD and Congress.
He said there had been no talks with RJD or Congress on the subject including seats on which each parties would contest.
"I am going to Delhi tonight and may talk to leaders of these parties," he said.
RJD Legislature party leader Abdul Bari Siddiqui also favoured alliance of like-minded parties against BJP in the bypoll.
"RJD is against BJP spreading its influence in Bihar and hence ready to join hands with like-minded parties to check there growth here," Siddiqui told reporters.
Asked about the number of seats the party would contest and who would lead the alliance, Siddiqui said, "These are minor issues which can be sorted out once the alliance is forged."
BJP described coming together of JD(U), RJD and Congress as "unholy alliance" and said people would not accept them.
"People of Bihar have already rejected these parties in Lok Sabha election and hence there coming together against BJP will yield no result," senior BJP leader and leader of Opposition in Assembly Nand Kishore Yadav told reporters.