North Korean leader Kim Jong-Un ordered preparations Friday for strategic rocket strikes on the U.S. mainland and military bases after U.S. stealth bombers flew training runs over South Korea.

The order came as U.S. Defense Secretary Chuck Hagel, with tensions soaring on the Korean peninsula, said Washington would not be cowed by Pyongyang's bellicose threats and stood ready to respond to "any eventuality".

Defense Secretary Chuck Hagel said Thursday that the United States was "prepared to deal with any eventuality" from North Korea and warned that danger from Pyongyang was on the rise.

"We will unequivocally defend -- we are unequivocally committed -- to that alliance with South Korea as well as our other allies in that region of the world," Hagel told reporters.

U.S. Defense Secretary Chuck Hagel promised "unwavering" support from the United States to its ally South Korea on Wednesday after North Korea severed a military hotline with Seoul.

During a call with his South Korean counterpart Kim Kwan-Jin, Hagel "reaffirmed the strength of the alliance, which has been, and continues to be, instrumental in maintaining stability on the Korean peninsula," Pentagon spokesman George Little said in a statement.

A Chinese border province has announced plans to beef up transport and trade links with North Korea, including new railways to the frontier, soon after pledges by Beijing to implement tough new U.N. sanctions.

China has been North Korea's sole major ally for decades and is overwhelmingly its biggest trade partner, but has come under increasing pressure to join in global sanctions aimed at punishing Pyongyang for repeated nuclear and missile tests.

North Korea severed its military hotline with South Korea on Wednesday, suspending the last direct communication link between the two countries at a time of heightened military tensions.

The decision coincided with an announcement that the North's top political leadership would meet in the next few days to discuss an unspecified "important issue" and make a "drastic turn.”

North Korea's military put its "strategic" rocket units on a war footing Tuesday, with a fresh threat to strike targets on the U.S. mainland, Hawaii and Guam, as well as South Korea.

The move came as South Korea marked the third anniversary of the sinking of its naval vessel "Cheonan" by what Seoul insists was a North Korean submarine.

South Korean President Park Geun-Hye warned North Korea on Tuesday that its only "path to survival" lay in abandoning its nuclear and missile programs.

In a speech to mark the third anniversary of the sinking of a South Korean naval vessel by what Seoul insists was a North Korean submarine, Park called on Pyongyang to "change course" at a time of elevated military tension on the Korean peninsula.

A new South Korea-U.S. pact providing for a joint military response even to low-level provocation by North Korea offers an added deterrent at a time of elevated tension, the South said Monday.

The two allies signed the military agreement on Friday in a move likely to fuel fresh outrage in Pyongyang, which has spent the past few weeks denouncing joint South Korea-U.S. military exercises.

North Korean leader Kim Jong-Un has inspected a feared special forces unit, ordering them to strike at "lightening speed" once war breaks out, state media reported Saturday.

The inspection comes at a time of escalating tensions on the Korean peninsula, with multiple threats from the North of an armed response to joint South Korea-U.S. military drills and to U.N. sanctions imposed after its nuclear test last month.

U.S. officials involved in enforcing international sanctions against North Korea expressed optimism Friday that China was willing to cooperate to disrupt financing for Pyongyang's nuclear and missile programs.

"China is part of the solution and is working with us in a cooperative spirit," Dan Fried, the State Department's sanctions coordinator, told reporters.