(CNN) — North Korea warned Sunday that the upcoming US-South Korea military exercises are "reckless behavior driving the situation into the uncontrollable phase of a nuclear war."
Pyongyang also declared that its army can target the United States anytime, and neither Guam, Hawaii nor the US mainland can "dodge the merciless strike."
The messages in Rodong Sinmun, the official government newspaper, come a day before the US starts the Ulchi Freedom Guardian military exercises with South Korea.
Tensions between the US and North Korea have grown in recent weeks.
Just last week, Pyongyang said it had finalized a plan to fire four missiles toward the US territory of Guam. State media reported that leader Kim Jong Un would assess the US' next move before giving launch orders.
Kim would "watch a little more the foolish and stupid conduct of the Yankees," a North Korean statement said last week.
But US military and Trump administration officials said the 10-day military exercises set to begin Monday, would go ahead as scheduled.
The annual drills antagonize Pyongyang, which sees them as practice for an invasion. However, the US and South Korea maintain they are purely defensive.
"The Trump group's declaration of the reckless nuclear war exercises against the DPRK ... is a reckless behavior driving the situation into the uncontrollable phase of a nuclear war," Rodong Sinmun said, using the acronym for Democratic People's Republic of Korea, the nation's official name.
It described North Korea as the "strongest possessor" of intercontinental ballistic missiles capable of striking the US mainland from anywhere.
"The Korean People's Army is keeping a high alert, fully ready to contain the enemies. It will take resolute steps the moment even a slight sign of the preventive war is spotted," it said.
It did not provide any details on what it meant by "preventive war."
Both US Secretary of State Rex Tillerson and Secretary of Defense James Mattis said last week that the US was keeping military options on the table in dealing with North Korea.
Tillerson said peaceful diplomatic pressure was the preferred way to get Pyongyang to stop its testing of nuclear-capable ballistic missiles. But he added that the diplomatic approach "has to be backed with military threat" if North Korea chooses to move forward with destabilizing actions.
Mattis also made clear the US' willingness to use force if North Korea steps out of line.
"In close collaboration with our allies, there are strong military consequences if the DPRK initiates hostilities," he said.
Promise from South Korea's President
As tensions escalate, South Korean President Moon Jae-in promised his citizens last week there "will be no war on the Korean Peninsula ever again."
Moon, who took office in May, announced on his 100th day in office that US and South Korean policies are aligned on North Korea.
US President Donald Trump assured South Korea he would consult with them before making any military decisions on North Korea, according to Moon.
Moon said North Korea's development of nuclear weapons technology was "nearing" a red line, which he described as "completing an ICBM and weaponizing it with a nuclear head."
North Korea claims it has successfully miniaturized a nuclear weapon. While some experts believe it may have the technology, others caution that even if it doesn't, North Korea should be taken at its word.
"If North Korea provokes again, it will face with much harsher sanction and won't stand it in the end. I want to warn North Korea to do no more dangerous gambling," Moon said.
His comments about averting war echoed similar statements he made Tuesday that only South Korea could give consent to initiate any conflict with the North.
"The government, putting everything on the line, will block war by all means," Moon said.
China weighs in
China has urged both Washington and Pyongyang to tone down the rhetoric and stop actions that inflame tensions, missile testing on North Korea's side and military exercises on the US and South Korean side.
China's Global Times newspaper, a state-run tabloid, was scathing of South Korea's decision to proceed with the drills.
"The drill will definitely provoke Pyongyang more, and Pyongyang is expected to make a more radical response," it said in an editorial.
"If South Korea really wants no war on the Korean Peninsula, it should try to stop this military exercise."
Euan McKirdy and Ben Westcott contributed to this report