Jofi Joseph is no longer working for the administration, a White House official confirmed. That official would not comment further on personnel matters.
Joseph took full responsibility for the Twitter account in a statement to Politico. He said it started as a "parody account" and apologized to those he insulted.
The postings on the NatSecWonk account, which is no longer available on Twitter, often took aim at administration policy and figures, including Secretary of State John Kerry and Defense Secretary Chuck Hagel, along with members of Congress.
One tweet said: "More people should be asking why John Kerry installed two former aides, both with ZERO foreign policy experience, into top posts at State."
Another tweet said: "That Obama only called Kerry/Hagel AFTER he made decision with his WH aides on going to Hill underscores how all foreign policy is WH-based." The tweet referred to Obama's surprise decision in late August to seek congressional authorization for military strikes against Syria as punishment for a chemical weapons attack in August.
Joseph began working at the State Department in 2009. Deputy State Department spokeswoman Marie Harf said he was assigned to the National Security Council in 2011 and became an NSC employee last August.
White House spokesman Jay Carney said White House staffers cannot access social media sites like Twitter from the White House unless they have an official, authorized account. Carney and many other senior administration officials have official Twitter accounts and often send many tweets a day.
The Associated Press contributed to this report.