SEOUL, South Korea (AP) -- The powerful sister of North Korean leader Kim Jong Un ripped South Korea for proceeding with military exercises with the United States that she said are an invasion rehearsal and warned Tuesday that the North will work faster to strengthen its preemptive strike capabilities.

Kim Yo Jong's statement came after South Korean media reported that the allied militaries will begin four days of preliminary training on Tuesday before holding computer-simulated drills on Aug. 16-26.

Kim said she was given authority to release the statement, implying the message came directly from her brother.

Hours after her comments were published on state media, South Korea's defense and unification ministries said North Korean officials hadn't responded to their calls Tuesday afternoon over inter-Korean hotlines, which had been disconnected for a year before North Korea agreed to reopen them in late July. The Koreas then described the move as a conciliatory gesture.

South Korean officials said they were closely monitoring the development, but that it wasn't immediately clear whether North Korea had cut the communication channels again.

Kim Yo Jong described South Korea's decision to hold joint exercises despite earlier warnings by the North as “perfidious behavior” that will push the allies into facing a “more serious security threat.”

She said continuing the drills exposed the hypocrisy of the Biden administration's offers to resume dialogue over North Korea's nuclear weapons program. She said a stable peace won't be achieved on the Korean Peninsula unless the United States withdraws its troops and weapons from the South.

Kim said North Korea will “put more spur to further increasing the deterrent of absolute capacity to cope with the ever-growing military threats from the U.S.,” including its capabilities for national defense and “powerful” preemptive strikes for “rapidly countering any military actions against us.”

“(The drills) are the most vivid expression of the U.S. hostile policy towards (North Korea), designed to stifle our state by force, and an unwelcoming act of self-destruction for which a dear price should be paid as they threaten the safety of our people and further imperil the situation on the Korean Peninsula,” Kim said.

“For peace to settle on the peninsula, it is imperative for the U.S. to withdraw its aggression troops and war hardware deployed in (South) Korea. As long as the U.S. forces stay in (South) Korea, the root cause for the periodic aggravation of the situation on the Korean Peninsula will never vanish.”

It wasn't immediately clear whether North Korea's threat to advance its preemptive strike capabilities signaled a resumption of testing activity.

North Korea ended a yearlong pause in ballistic tests in March by firing two short-range missiles into the sea, continuing a tradition of testing new U.S. administrations with weapons demonstrations and other provocations apparently aimed at measuring Washington's response and wresting concessions.

But North Korea hasn't conducted any known test launches since then as Kim Jong Un focused national efforts on fending off the coronavirus and salvaging a broken economy damaged further by pandemic border closures.

North Korea's angry reaction to the drills further diminishes South Korean hopes for improving bilateral ties, which rose after the North agreed to reopen long-stalled communication channels with the South.

But just days after the lines were restored, Kim Yo Jong warned that the planned military drills between South Korea and the United States will undermine prospects for better inter-Korean ties.

Some analysts say North Korea's decision to restore the communication lines was mainly aimed at pushing Seoul to convince Washington to make concessions while nuclear diplomacy remains deadlocked.

“Kim Yo Jong's threatening statement demonstrates that North Korea will use even restrained U.S.-South Korea defense exercises as an excuse not to implement inter-Korean cooperation agreements and to justify its next military provocation,” said Leif-Eric Easley, a professor of international studies at Ewha Womans University in Seoul.

Kim Jong Un has pledged to bolster his country's nuclear deterrent while urging his people to stay resilient in a struggle for economic self-dependence in the face of U.S.-led pressure. His government has so far rejected the Biden administration's overture for talks, demanding that Washington abandon its “hostile” policies first.

The United States keeps about 28,000 troops in South Korea to help deter potential aggression from North Korea, in a legacy of the 1950-53 Korean War. The allies have yet to officially announce the details of this month's drills.

Boo Seung-Chan, spokesperson of South Korea's Defense Ministry, said in a briefing that the allies were discussing the “timing, scale and methods” of the summertime drills.

North Korea has long bristled at joint military exercises between South Korea and the United States, which the allies describe as defensive in nature, and often responds to them with its own weapons tests.

In the past few years, however, South Korea and the United States have canceled or downsized some of their training to support now-dormant diplomacy aimed at ending the North Korean nuclear crisis or because of the COVID-19 pandemic.

North Korea has suspended its nuclear and long-range missile tests since 2018, when leader Kim Jong Un initiated diplomacy with South Korea and then-President Donald Trump while attempting to leverage his nuclear weapons for badly needed sanctions relief.

After the talks fell through in 2019, North Korea ramped up tests of new short-range, solid-fuel weapons to improve its ability to deliver nuclear strikes and overwhelm missile defense systems in South Korea and Japan.

Inter-Korean ties flourished during the diplomacy of 2018, during which Kim and South Korean President Moon Jae-in met three times and vowed to resume inter-Korean economic cooperation when possible, expressing optimism that the sanctions would end and allow such projects.

But North Korea later cut off ties with South Korea following the collapse of the second summit between Kim and Trump in 2019 when the Americans rejected North Korea's demand for major sanctions relief in exchange for a partial surrender of its nuclear capabilities.