(AP) -- Defense Secretary Chuck Hagel announced Monday he is stepping down, leaving under pressure following a rocky tenure in which he has struggled to break through the White House's insular team of national security advisers.

During a White House ceremony Monday, Obama said he and Hagel had determined it was an "appropriate time for him to complete his service."

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

The president praised Hagel, a Republican who grew close to Obama while they both served in the Senate, as an "exemplary defense secretary" who forged a strong bond with troops stationed around the world. Hagel, who served in the Vietnam War, is the first enlisted combat veteran to serve as defense secretary.

"Chuck Hagel has devoted himself to our national security and our men and women in uniform across six decades," Obama said.