South Korea and the United States on Thursday warned Pyongyang against a potential missile test that they said would be a violation of U.N. resolutions as the two countries wrapped up annual high-level defense talks.

There has been widespread speculation that North Korea is planning a satellite rocket launch to mark the 70th anniversary of the ruling Workers' Party on October 10.

North Korean leader Kim Jong-Un put his frontline troops on a war footing Friday to back up an ultimatum for South Korea to halt high-decibel propaganda broadcasts across the border.

The move came as military tensions on the divided Korean peninsula soared following a rare exchange of artillery fire on Thursday that put the South Korean army on maximum alert.

North Korea on Thursday condemned a looming South Korea-U.S. joint military exercise as a "declaration of war" and boasted of its ability to make retaliatory strikes against Seoul and the White House.

The annual two-week "Ulchi Freedom" exercise, which kicks off Monday, involves tens of thousands of troops in what is a largely computer-simulated rehearsal for a North Korean invasion.

South Korea on Monday accused North Korea of planting landmines that maimed two soldiers on border patrol, ramping up military tensions as it threatened to make Pyongyang pay a "harsh price".

The Defense Ministry said it believed three landmines exploded in the incident last Tuesday, hitting a patrol in the demilitarized zone (DMZ) -- a buffer zone stretching two kilometers on either side of the actual frontier line dividing the two Koreas.

Tokyo voiced its frustration on Friday a year after North Korea said it had started re-investigating the fate of Japanese kidnapped by Pyongyang's spies decades ago, with no new leads in sight.

"It is extremely regrettable that no abduction victims returned home although it's been a year since the investigation started," Prime Minister Shinzo Abe said during a parliamentary session. 

North Korea Tuesday sentenced two South Korean men to hard labor for life on espionage and other charges, state media reported, rejecting repeated calls by Seoul to release them.

The North's Supreme Court handed down life sentences on charges of spying for South Korea's intelligence agency, said Pyongyang's official Korean Central News Agency (KCNA).

North Korea on Wednesday repatriated two South Koreans in their 50s who were detained last month for illegally entering the communist state, Seoul's Unification Ministry said.

The two -- a 59-year-old man and a 51-year-old woman -- were reported missing while travelling in northeastern China near the border with North Korea.

North Korean leader Kim Jong-Un has hailed the firing of a new anti-ship rocket as "another fresh milestone" in bolstering the country's naval power following the testing of a submarine-launched ballistic missile (SLBM).

Kim watched the test of the "highly intelligent" rockets to be deployed at naval units, Pyongyang's KCNA news agency reported on Monday.

South Korea and the United States kicked off a major anti-submarine drill on Monday, weeks after North Korea claimed it had successfully tested a submarine-launched ballistic missile.

The three-day exercise, aimed at tackling provocations by North Korean submarines, is being conducted off the southern island of Jeju, the South's defense ministry said.

Nuclear envoys from South Korea, Japan and the United States agreed Wednesday to step up pressure -- including possible sanctions -- on North Korea to abandon nuclear weapons, while keeping diplomatic options "open."

Speaking to reporters after trilateral talks in Seoul, the three officials also highlighted the "seriousness" of the progress the North has been making with its expanding nuclear and ballistic missile programs.