North Korea's defence ministry accused the United States and South Korea of ramping up reconnaissance activities around the inter-Korean border, warning that it will act if its sovereignty and security is violated, state media KCNA said on Sunday, Ednews informs via Reuters.

North Korea's vice defence minister Kim Gang Il said the US had flown at least 16 of its RC-135 and U-2S strategic reconnaissance planes and RQ-4B drone over the Korean peninsula between May 13 and 24.

He also accused South Korea's navy and coast guard of stoking military tension by stepping up patrol activities and increasingly breaching the maritime border.

Kim also criticised propaganda leaflets sent in balloons from South Korea, calling it a "dangerous provocation."

North Korean defectors and activists in South Korea regularly send balloons containing anti-Pyongyang leaflets, alongside food, medicine, money, mini radios and USB sticks loaded with South Korean news and dramas.

"Even now, the US and South Korean puppet air forces are continuously mobilising various aircraft with little or no time gap throughout the day, carrying out aerial reconnaissance activities at a level comparable to wartime situations," Kim said in a statement published by KCNA.

Those activities seriously infringe on North Korea's national sovereignty and security, and will not be met with "offensive" countermeasures, he said.

"We will act immediately when the nation's sovereignty and security interests are violated," Kim added.