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.

North Korea's vice premier Choe Yong-Gon has been executed for voicing frustration at the policies of leader Kim Jong-Un, South Korea's Yonhap news agency said Wednesday, citing an anonymous source.

Choe, who took the job in June 2014, was executed by firing squad in May after voicing opposition to forestry policies promoted by Kim, Yonhap said, citing the source "with knowledge of the North".

South Korea ramped up border security Tuesday as military tensions flared following landmine blasts blamed on North Korea, and the presidential office in Seoul demanded a formal apology.

South Korea says North Korean soldiers sneaked across the border and laid the mines, three of which were tripped by members of a South Korean border patrol on Tuesday last week.

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.

The widow of late South Korean president Kim Dae-Jung returned home Saturday from a rare trip to North Korea but her hoped-for meeting with leader Kim Jong-Un failed to come off.

Lee Hee-Ho's four-day visit was ostensibly humanitarian but there had been much speculation about a possible meeting with Kim, who personally invited her.

A U.S. citizen working as a Christian missionary in Thailand could face up to five years in jail on charges of smuggling seven North Koreans into the kingdom, police said Wednesday.

Hundreds of North Koreans risk their lives each year to escape poverty and repression in the hardline communist nation.

South Korean President Park Geun-Hye offered North Korea a "door of opportunity" as she flagged off restoration work Wednesday on a railway line that could one day link both Koreas with Europe via the Trans-Siberian.

The work will focus on the South's section of a disconnected inter-Korean railroad that once linked central Seoul with the North Korean eastern city of Wonsan.

North Korea said Thursday that stalled dialogue over its nuclear program was down to aggressive U.S. military exercises and accused Washington of seeking to shift blame onto Pyongyang.

"It is the height of shamelessness and hypocrisy that the U.S. is trumpeting about 'will for dialogue' and 'flexibility' while resorting to military maneuvers ceaselessly," a foreign ministry spokesman said.

Fresh satellite images suggest North Korea has completed upgrades at its main satellite launch site, fueling speculation of a long-range rocket launch to coincide with a major political anniversary in October.

The U.S.-Korea Institute at Johns Hopkins University said Wednesday that analysis of the new pictures showed that construction begun in spring 2015 at the Sohae launch center had been concluded.

A North Korean ambassador said in New York on Tuesday that Pyongyang might launch a new missile test in October.

Any rocket launch would almost certainly be viewed by the international community as a disguised ballistic missile test and result in the imposition of fresh sanctions.