The United States has no "monopoly" on nuclear weapons, the mouthpiece of North Korea's ruling Workers' Party warned on Friday as Pyongyang stepped up its rhetoric ahead of the annual joint U.S.-South Korean military exercises, Reuters reported.

"The DPRK will wage a merciless sacred war against the (United States) now that the latter has chosen confrontation,"said the country's official KCNA news agency, quoting an article in Rodong Sinmum, the ruling Workers' Party newspaper."The (United States) is seriously mistaken if it thinks its mainland is safe."

DPRK stands for "Democratic People's Republic of Korea," North Korea's official name.

U.S. and South Korean forces noted on Tuesday that they would begin eight weeks of joint military drills starting March 2, an announcement that is believed to have triggered the verbal assault. The United States was "much upset by the fact that there may be a sign of detente on the Korean peninsula, thanks to the DPRK's initiative and efforts to achieve peace this year," Rodong Sinmun claimed.

North Korea's nuclear arsenal, meanwhile, could expand to up to 100 warheads by 2020, the New York Times said based on a new research report. That estimate from experts on the Hermit Kingdom well exceeds past figures and "should force renewed attention on a threat that has been eclipsed by other crises," the newspaper argued.

Talks between the United States, the four remaining permanent members of the U.N. Security Council and Germany with Iran appeared to be close to producing a framework agreement on the Islamic Republic's nuclear ambitions earlier this week, the New York Times recalled. The six-party talks between China, Japan, Russia, South Korea, the United States and North Korea, however, seem to have stalled.

Nevertheless, Wendy Sherman, the U.S. undersecretary of State for political affairs, said international pressure on Pyongyang was growing, according to Reuters.

The North Korean regime was increasingly isolated and trying "to conceal a weak hand ... through a clenched fist," Sherman recently told an audience at Washington's Carnegie Endowment.

The Obama administration, meanwhile, is open to new talks with North Korea as long as they are focused on the country's nuclear capabilities.

"There are other things that can be discussed," she said. "But at the core is the nuclear program."