WASHINGTON (AP) — Ash Carter, sworn in Tuesday as President Barack Obama's fourth secretary of defense amid multiple global threats, pledged to offer his most candid strategic advice and carefully consider decisions about sending troops into harm's way.

Hours after taking the oath of office, Carter met with Obama and Vice President Joe Biden in the Oval Office to discuss the security challenges that await him in the new job. Obama said their discussion covered battles in Ukraine, the threat from Islamic State militants and countering violent extremism that he said has been "turbocharged through the Internet."

"I could not be more confident that Ash Carter is going to do an outstanding job as secretary of defense, and he is hitting the ground running," Obama said, citing Carter's experience in other leadership positions at the Pentagon.

Carter, 60, replaces Chuck Hagel, who resigned under pressure after a rocky relationship with the White House. In an email to all Defense Department personnel, Carter wrote that the challenging times "demand leadership and focus."

"In addressing these challenges, I have pledged to provide the president my most candid strategic advice," Carter wrote.

"I pledge to make decisions about sending you into harm's way with the greatest reflection and utmost care — because this is my highest responsibility as secretary of defense," Carter's memo read.

Carter's swearing-in ceremony was held in the White House Roosevelt Room Tuesday morning while most of the federal government was closed because of snow. Biden issued the oath of office on a Bible held by Carter's wife.

Bide said Carter faces "many tough missions," ranging from battling Islamic State militants and strengthening the NATO alliance, to technological advancements and budget cuts. "This is the guy that fits the job," Biden said, calling Carter a "profoundly capable manager."

Carter responded that he was taking office with three commitments. He pledged to help Obama make the best possible decisions about U.S. and global security; to protect the dignity, safety and well-being of service members; and to build a force for the future that embraces change.

With lawmakers debating Pentagon funding levels, Carter said he was committed to "not only securing the resources we need but to make sure we make the best use of the taxpayer's dollar."

The ceremony was attended by Carter's son, William, and a handful of senior military leaders, including Deputy Defense Secretary Bob Work and Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff.

Carter declined to answer a question about providing arms to Ukraine before he left the room.

Biden teased Carter, who previously held other positions in Pentagon leadership, for a habit of holding meetings while "walking briskly down the hallways of the Pentagon, with his aides struggling to scribble and jog simultaneously."

"Ash Carter's a thinker and a doer," Biden said. The vice president said Carter "saved thousands of lives and limbs" by getting protective undergarments and mine-resistant vehicles for troops in Iraq and Afghanistan.

Carter had earlier reported to work at the snowy, icy Potomac River entrance to the Pentagon. His wife, Stephanie, slipped and fell on the icy pavement before the gathered media, but laughed it off and later told journalists at the White House she was fine. Biden kept his hands on her shoulders as her husband addressed reporters at the swearing-in, at one point leaning down close to her ear.

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AP videojournalist Sagar Meghani and reporter Lolita C. Baldor contributed to this report.

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