He refused to join issue with Congress vice president Rahul Gandhi, who accused him of having returned to "communal forces" for "selfish" political motives, saying he would respond those attacking him at the "right time".
"I took the decision to form the new government in the interest of Bihar ... I am committed to the development of Bihar," Kumar told reporters after taking oath.
Kumar is facing a barrage of criticism from RJD chief Lalu Prasad and a host of leaders of the erstwhile Grand Alliance for dumping the coalition and joining hands with NDA to form the government.
"There will be no compromise on corruption," an official statement quoted Kumar as saying.
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"I am confident that the pace of progress of the state will gain fresh momentum with assistance from the Centre," he added.
Meanwhile, JD(U) leaders came down heavily on Lalu for attacking Kumar in connection with a murder case registered with Pandarak police station in Patna district in 1991.
JD(U) general secretary R C P Singh, senior leader Rajiv Ranjan Singh Lallan and Neeraj Kumar held a press conference to hit out at Prasad, who had yesterday referred to a pending case against Nitish Kumar as a counterattack after he walked out of the grand alliance.
Lallan Singh said Lalu was in the habit of speaking "half truths" and "twisting facts".
Giving details of the status of the case, he said five people were accused in it. Lalu Prasad was then the Bihar chief minister. Police had probed the case, and on January 31, 1993, a charge sheet was filed against three accused - Dilip Singh, Yogendra Yadav and Baudhu Yadav. Kumar was found innocent.
Yogendra Yadav, who was among three charge-sheeted, had challenged it in Patna High Court which had stayed the case in the Barh court on April 22, 2009.
"A man Ashok Kumar Singh, who is neither a complainant nor a witness in the case, had on January 20, 2009 filed a case in the court of additional chief judicial magistrate of Barh to register his protest against Kumar being exonerated. The Barh ACJM in violation of the Patna high court's stay took cognisance of the case on August 20, 2009 and issued summons to Nitish Kumar," Lallan Singh said.
The Barh ACJM's reply was not accepted by the HC, which asked him to file another and the case is now pending before it, he said.
"Ashok Singh had recorded his statement under section 164 (made before an metropolitan or judicial magistrate whether or not he has jurisdiction in the case) that his signature was forcibly taken on a blank piece of paper to file a case against Kumar's acquittal," Singh claimed.