1st LD: Ashton Carter sworn in as Obama's new Defense Secretary

Former Deputy Defense Secretary Ashton Carter was sworn in Tuesday as the 25th U.S. Defense Secretary, the fourth defense chief in U.S. President Barack Obama 's six years in office.

After taking the helm, the new defense chief will have to tackle mounting issues, such as the fight against the extremist group Islamic State (IS) and a decision on whether to arm the Ukrainian forces with lethal weapons.

Carter's inauguration also came at a time when the Yemeni government, a key U.S. Middle East partner in fighting al-Qaeda, was on the verge of collapse, threatening U.S. counter-terrorism strategy across the region.

At his Feb. 4 Senate Armed Service Committee hearing, Carter described the work that lies ahead for him and Pentagon as unprecedented.

"I think we are in a time where the number and severity of risks is something I've not seen before in my life," he said.

Another challenge for Carter will come from the U.S. Congress on the 2016 Pentagon budget. The Obama administration recently sought a base defense budget of 534 billion dollars when it sent its 2016 spending request to Congress, 35 billion dollars more than a federal budget cap imposed in 2011. Endite