Roundup: New U.S. defense chief sworn in amid mounting challenges home and abroad

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

A trained physicist and a longtime Pentagon bureaucrat, Carter took the helm at a time when the military are facing mounting issues, such as redefining military strategies against the extremist group Islamic State (IS) and a decision on whether to arm the Ukrainian forces with lethal weapons.

"Our first priority is helping the president make the best possible national security decisions for protecting our country and then implementing those decisions," said Carter in a statement after his inauguration ceremony.

Although Obama's move on Feb. 11 to seek authorization to use force against IS from Congress ensured "flexibility" Carter has reportedly asked for, initial reactions from lawmakers on both sides of the aisle indicated a tough road ahead, with Republican lawmakers saying the draft put too many restraints which would handcuff the military, while their Democratic counterparts are worried that fewer limits would not prevent Obama from committing more ground troops in the future.

In his first statement after assuming office, Carter said he would provide the president with the "most candid strategic advice. " However, his preference for equipping Kiev with lethal weapons may be the first collision point between Pentagon and the White House.

At the U.S. Senate Armed Service Committee hearing on Feb. 4, Carter said he was "very much inclined" to support arming lethal weapons to Kiev despite the White House's efforts to play down the possibility. In response, White House spokesman Josh Earnest stressed that such decision would ultimately fall to Obama as commander-in-chief.

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.

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

Carter is expected to testify on Capitol Hill about the budget request after Congress's week-long President's Day recess. He has warned earlier that if Congress fails to raise the cap, U.S. security strategy would go cheap in the future.

"We must steer through the turmoil of sequestration, which imposes wasteful uncertainty and risk to our nation's defense," said Carter in his statement.

Despite deep division between the Obama administration and the Republican-controlled Congress over foreign priorities and military strategies, the nomination of Obama's pick for the new Pentagon chief encountered few setbacks, and his confirmation won bipartisan support.

However, Obama's pick for the attorney general, Loretta Lynch, was having a hard time winning support from Congressional Republicans, who have expressed concerns about Lynch's stance on Obama's disputed executive order on immigration last year. Endite