North Korea threatened Monday to resume propaganda loudspeaker broadcasts and deploy more weapons along the border with South Korea, as the rivals remain locked in animosities over last month’s mine blasts that injured three South Korean soldiers.

Earlier this month, South Korea formally blamed North Korea-planted mines for the Sept. 21 blasts at the border, urging the North to apologize and remove other mines.

That announcement came after North Korea denied responsibility for the explosions and warned of strong retaliation if provoked.

On Monday, Kim Yo Jong, the powerful sister of North Korean leader Kim Jong Un, accused South Korea of firing warning shots and issuing warning broadcasts numerous times at the border since August under “such an unreasonable pretext” that North Korean soldiers crossed the border.

She said that such provocations “have gone beyond the red line” since the mine blasts.

“A grave situation is prevailing along the southern border due to the reckless behavior of the ROK military gangsters,” Kim Yo Jong said, referring to South Korea by its official name, the Republic of Korea.

She said the North Korean military was considering “deploying more tactical strike means along the southern border as a preemptive and suppressive measure to deal with contingency, and of resuming the loudspeaker broadcasting against the ROK.”

In recent years, tensions along the Korean border, one of the world’s most heavily fortified, have increased as North Korea added mines, barbed wire fences, anti-tank barriers and other front-line fortifications in line with its policy of terminating ties with South Korea.

South Korea’s military earlier said North Korean soldiers violated the border’s military demarcation line 50 times this year during their fortification works. South Korea’s military said it responded by firing warning shots and making warning broadcasts. It didn’t immediately respond to the latest North Korean warning.

In 2015, mine blasts blamed on North Korea that maimed two South Korean soldiers pushed the Koreas to the brink of an armed conflict, after South Korea resumed front-line loudspeaker propaganda broadcasts for the first time in 11 years.

Loudspeaker broadcasts are one of the old-fashioned, Cold War-era psychological warfare campaigns the two Koreas have occasionally used against each other in times of confrontation.

Since taking office last year, South Korea’s liberal government, led by President Lee Jae-Myung, halted such broadcasts as part of efforts to improve ties with North Korea. North Korea later stopped its own broadcasts.

North Korea has recently dialed up weapons testing activities in a likely bid to pressure the United States to make bigger concessions, after President Donald Trump renewed his efforts to restore diplomacy with Kim Jong Un.