South Korea and the United States kicked off their largest-ever joint military drills on Monday, Yonhap news agency reported. 

Participation in the joint drills -- known as Key Resolve and Foal Eagle -- has been bumped up this year to involve 300,000 South Korean and 15,000 US troops, as well as strategic US naval vessels and air force assets. 

In a statement issued hours before the exercises began, North Korea's powerful National Defence Commission said it was prepared for an "all-out" military counter-offensive. 

"As the joint military exercises to be staged by the enemies are regarded as the most undisguised nuclear war drills aimed to infringe upon the sovereignty of (North Korea), its military counteraction will be more pre-emptive and offensive nuclear strike to cope with them," it said. 

The threat came just days after leader Kim Jong-Un ordered the country's nuclear arsenal to be placed on standby, in response to the sanctions resolution adopted last week by the UN Security Council.

Source: (MENA