North Korea leader Kim Jong-un and Russian President Vladimir Putin have exchanged letters pledging to develop their ties into what Kim called a "long-standing strategic relationship", Pyongyang's state media KCNA says. The letters mark the 78th anniversary of Korea's liberation from Japan's 1910-45 colonial rule, which is also celebrated as a national holiday in South Korea. In his letter to Putin on Tuesday, Kim said the two countries' friendship was forged in World War II with victory over Japan and is now "fully demonstrating their invincibility and might in the struggle to smash the imperialists' arbitrary practices and hegemony", KCNA said. "I am firmly convinced that the friendship and solidarity ... will be further developed into a long-standing strategic relationship in conformity with the demand of the new era," Kim was quoted as saying in the letter. "The two countries will always emerge victorious, strongly supporting and co-operating with each other in the course of achieving their common goal and cause." The United States has accused North Korea of providing weapons to Russia for its war in Ukraine, including artillery shells, shoulder-fired rockets and missiles. Pyongyang and Moscow have denied any arms transactions. Last month, Russia's defence minister stood shoulder to shoulder with Kim as they reviewed North Korea's newest nuclear-capable missiles and attack drones at a military parade in Pyongyang. Washington remains "deeply concerned" about North Korea aiding Russia's war effort in Ukraine and believes Moscow is seeking to increase its co-operation with Pyongyang, State Department deputy spokesman Vedant Patel said on Tuesday. "Any kind of security co-operation or arms deal between North Korea and Russia would certainly violate a series of UN Security Council resolutions," Patel said at a regular press briefing. Putin, in his message to Kim, also pledged to bolster bilateral ties. "I am sure that we will strengthen the bilateral co-operation in all fields for the two peoples' well-being and the firm stability and security of the Korean peninsula and the whole of Northeast Asia," Putin said, according to KCNA. The leaders of South Korea, the United States and Japan are set to discuss security co-operation over North Korea, Ukraine and other issues at a trilateral summit on August 18 at Camp David. Australian Associated Press