Washington, Feb 17 (IANS): Ashton Carter was sworn in Tuesday as the 25th US defense secretary -the fourth in US President Barack Obama's six years in office.

After taking the helm, he 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 US Middle East partner in fighting Al Qaeda, was on the verge of collapse, threatening US counter-terrorism strategy across the region, according to Xinhua.

At his Feb 4 Senate Armed Service Committee hearing, Carter, a former deputy defense secretary 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 US Congress on the 2016 Pentagon budget. The Obama administration recently sought a base defence budget of $534 billion when it sent its 2016 spending request to Congress, $35 billion more than a federal budget cap imposed in 2011.