North Korea fired two short-range ballistic missiles toward the East Sea on Monday, Seoul's military said, a day after South Korea and the United States staged joint air drills, involving B-1B bombers, in response to the North's long-range missile launch.

The Joint Chiefs of Staff said it detected the launch from the Sukchon area in South Pyongan Province between 7 a.m. and 7:11 a.m. It did not immediately provide details.

"While strengthening its monitoring and vigilance, our military is maintaining a full readiness posture in close cooperation with the US," the JCS said in a text message sent to reporters.

Monday's launch marks the North's third missile provocation this year.

About an hour after the JCS announcement, North Korea said its military carried out firing drills involving multiple rocket launchers at 7 a.m.

An artillery unit of the Korean People's Army launched two rounds of shots from the 600-mm rocket launch system toward the East Sea, according to the North's official Korean Central News Agency.

The North described the "super-large" rocket launcher as a means of "tactical nuclear attack," stressing that its latest firing drills reinforced the KPA's readiness to respond to the allies' air force power.

In a ceremony to "present" the 600-mm rocket launcher at a key party meeting in late December, North Korean leader Kim Jong-un called it an "offensive weapon" capable of carrying tactical nuclear warheads that could put the entire South Korea within its range.

The latest saber-rattling raised concerns that the North may continue to engage in such provocations as the allies plan to hold a tabletop military exercise against North Korean nuclear threats this week and their springtime Freedom Shield exercise next month.

The allies carried out the air drills, involving the US bombers and South Korean F-35A stealth jets, on Sunday as the North launched what it claims to be a Hwasong-15 intercontinental ballistic missile on Saturday.

Kim Yo-jong, the influential sister of North Korean leader Kim Jong-un, issued another sharp-tongued threat of "corresponding" actions against the allies' military drills.

"The frequency of using the Pacific as our firing range depends upon the US," she said in an English-language statement carried by the KCNA.

She said the North is "carefully" examining the impact of the deployment of US strategic assets on its security, vowing to "take corresponding counteraction" if the move is judged to pose any "direct or indirect" threat to the North. (Yonhap)