The Obama administration bluntly warned North Korea on Monday that it will use military force if necessary to protect the United States and its allies in Asia from a North Korean nuclear strike or to prevent the impoverished North from selling nuclear weapons or expertise.

Washington announced fresh sanctions against North Korea on Monday amid rising tension on the Korean Peninsula. The U.S. military also began annual joint exercises with ally South Korea, over objections from North Korea and an announcement that it would nullify the 60-year armistice that ended the Korean War.

“Recently, North Korean officials have made some highly provocative statements,” White House national security adviser Thomas E. Donilon told the Asia Society in New York. “North Korea’s claims may be hyperbolic, but as to the policy of the United States, there should be no doubt.”

“We will draw upon the full range of our capabilities to protect against, and to respond to, the threat posed to us and to our allies by North Korea,” he said.

Donilon added that while the United States would like better relations with North Korea, it “refuses to reward bad North Korean behavior.”

“The United States,” he said, “will not play the game of accepting empty promises or yielding to threats.”

The United States is one of five nations that have sought unsuccessfully to negotiate a buyout of the North Korean nuclear program. The North has tested three nuclear devices and is assumed to be gradually perfecting the technology to deliver a nuclear warhead far from its own shores.

On Monday, the rising tensions on the Korean Peninsula were underscored by the North’s announcement that it had “completely scrapped” the 1953 armistice agreement.

The armistice has kept a shaky peace on the peninsula for 60 years, and the North’s apparent withdrawal — coupled with its severing of a communications hotline at the demilitarized border Monday — makes it more difficult for South Korea and the United States to prevent or resolve disputes with Pyongyang.

North Korea has made several similar announcements in the past, most recently in 2009, and analysts cautioned this latest declaration could prove to be bluster. Experts also noted that Pyongyang — whether bound by the cease-fire or not — has occasionally ignored its terms, most notably with fatal attacks on the South in 2010.

Still, anxiety about the North is particularly high now for the United States and its allies because they have little insight into the decision-making style of Kim Jong Eun, the young leader who took power of the opaque police state in December 2011 and now appears to be using the same brand of brinkmanship his father once did.

Kim on Thursday visited troops on several Yellow Sea islets off North Korea’s southern coast, not far from a South Korean island that was shelled in 2010. According to a state media account of the visit, Kim “reconfirmed in detail reinforced firepower strike means and targets of the enemy.”

“They’re giving all the motions to make us believe that some sort of provocation is coming,” said Ken Gause, an expert on North Korean leadership at CNA, an Alexandria-based organization. “Provocations are not necessarily imminent, but the probability is higher . . . and the ability to manage a provocation is much more difficult if they have withdrawn from the communication channels that existed under the armistice.”

Gause added that the North’s latest threats were more specific than usual — an escalation that has experts “scratching their heads about how willing they are to push the envelope.”

Many experts think that the North, by raising tensions and pulling away from existing lines of communication, is trying to pressure the United States into bilateral talks. In such talks, those experts believe, the North would seek a peace treaty, a major reduction of U.S. troops in the region and recognition as a nuclear state — a concession that Obama administration officials say they refuse to consider.

Denuclearization “is still the goal of U.S. policy,” Glyn Davies, the U.S. envoy for North Korea, told the Senate Foreign Relations Committee on Thursday.

The new sanctions unveiled Monday by the Obama administration target North Korea’s main foreign exchange bank and several of the country’s top economic leaders. The Foreign Trade Bank of North Korea was singled out because of its alleged support for companies linked to the country’s nuclear and ballistic missile programs, said David S. Cohen, undersecretary for terrorism and financial intelligence.

The sanctions freeze any U.S. assets that may be owned by the bank, and, more significantly, they serve as a warning to other countries that trade with North Korea, Cohen said. “We urge financial institutions around the world to be particularly wary of the risks of doing business” with the trade bank, he said.

Yoonjung Seo in Seoul and Joby Warrick in Washington contributed to this report.