Meanwhile, China - North Korea's closest ally and trading partner - expressed alarm at how far the rhetoric on both sides had gone, warning of the increased risk of a "fatal misjudgement"."The worldwide community won't accept North Korea as a nuclear power".
South Korean analysts say the nighttime flights, and also the decisions by Washington and Seoul to release the itinerary of the warplanes, are aimed at sending a clear warning to North Korea and demonstrating capability for surprise attacks.
The European Union on Tuesday strengthened measures on North Korea over missile tests, banning natural gas sales and imports of textiles.
A Pentagon spokesman, Colonel Robert Manning, said he was aware of the report, but declined to confirm or deny any aspect of it."I can assure you that we are confident in the security of our operations plans and our ability to deal with any threat from North Korea", Manning said."I am not going to address whether or not that [hack] has occurred".
According to South Korea's Joint Chiefs of Staff in a statement on Wednesday, the two U.S. Air Force B-1B bombers were joined by two F-15K fighters from the South Korean military after leaving their base in Guam.
At the end of last month, North Korea's foreign minister Ri Yong-ho said that the U.S. had "declared war" on the authoritarian state and was "right" to shoot down American planes. Pyongyang belatedly responded by relocating some of its military aircraft to its east coast, the National Intelligence Service then said.
Reports of reconnaissance on U.S. utilities follow earlier reports alleging DPRK spies stole a large cache of military documents from South Korea, including a plan to assassinate North Korea's leader, Kim Jong-un.
"There is an urgent need for the military to change and update parts that were stolen by North Korea", Lee said. Kim is the third generation of his family to rule North Korea.
Lee said that 235 gigabytes of military documents were taken, but the military has yet to identify 80 percent of the documents that were compromised.
The North reacted furiously when the United Nations approved the new measures, saying its response would make the United States suffer "the greatest pain it has ever experienced in its history". Other stolen data included contingency plans for South Korean special forces and information on military facilities and power plants, he said.