The United States announced sanctions on two of North Korea's most prominent officials behind its ballistic missile program on Tuesday, while Russia reiterated an offer to mediate to ease tension between Washington and Pyongyang.

The new US steps were the latest in a campaign aimed at forcing North Korea - which has defied years of multilateral and bilateral sanctions - to abandon a weapons program aimed at developing nuclear-tipped missiles capable of hitting the United States.

"Treasury is targeting leaders of North Korea's ballistic missile programs, as part of our maximum pressure campaign to isolate (North Korea) and achieve a fully denuclearized Korean Peninsula," Treasury Secretary Steven Mnuchin said in a statement.

The move followed new United Nations sanctions announced last Friday in response to North Korea's November 29 test of an ICBM that Pyongyang said put all of the US mainland within range of its nuclear weapons. Those sanctions sought to further limit North Korea's access to refined petroleum products and crude oil and its earnings from workers abroad.

North Korea declared the UN steps to be an act of war and tantamount to a complete economic blockade.

The standoff between the United States and North Korea has raised fears of a new conflict on the Korean peninsula, which has remained in a technical state of war since the 1950-53 Korean War ended in an armistice, not a peace treaty.

The US Treasury named the targeted officials as Kim Jong Sik and Ri Pyong Chol. It said Kim was reportedly a major figure in North Korea's efforts to switch its missile program from liquid to solid fuel, while Ri was reported to be a key official in its intercontinental ballistic missile (ICBM) development.