WASHINGTON (AP) -- Defense Secretary Chuck Hagel is stepping down from President Barack Obama's Cabinet, senior administration officials said Monday, following a tenure in which he has struggled to break through the White House's insular foreign policy team.

Hagel is the first senior Obama adviser to leave the administration following the sweeping losses for Obama's party in the midterm elections. It also comes as the president's national security team has been battered by multiple foreign policy crises, include the rise of the Islamic State militants in Iraq and Syria.

A senior defense official said that Hagel submitted his resignation letter to Obama on Monday morning and the president accepted it. Hagel agreed to remain in office until his successor is confirmed by the Senate, the official said.

The official said both Hagel and Obama "determined that it was time for new leadership in the Pentagon," adding that they had been discussing the matter over a period of several weeks.

Obama was to announce Hagel's resignation Monday. The president is not expected to nominate a new Pentagon chief Monday, according to one official.

The officials insisted on anonymity because they were not authorized to discuss the matter by name ahead of Obama's official announcement.

Hagel is a Republican who served as senator from Nebraska and became a critic of U.S. involvement in Iraq. Obama nominated him to succeed Leon Panetta as Defense Secretary in his second term.

Hagel served in the Vietnam War and received two Purple Hearts.

National Security Writer Robert Burns contributed to this story.