A declassified 1982 document shows that then-US President Reagan was briefed by top security aides that a Soviet nuclear attack on the United States could result in civilian casualties totaling about 80 million.

The document, made public recently by George Washington University's National Security Archives, was used by then National Security Advisor William Clark, Defense Secretary Caspar Weinberger and Joint Chiefs of Staff Chairman David Jones for a presidential briefing on the Soviet Union's capabilities and American vulnerabilities.

"We have two fundamental areas of concern here today. First, the character of a Soviet attack -- size, location and intent; and secondly, the capacity for a US response, with the emergency procedures to be used in the event of such an occurrence," the document said.

"There is no way of predicting how such an attack might come to pass. The Soviets could strike out of the blue, having decided that time was running against them and hoping that a forceful attack on our nerve centers would disable our ability to respond at all," it said.

According to the document, the top US security officials estimated civilian population casualties at about 80 million, which was about one-third of the US population at the time. The officials also reported that the casualties could be halved to 40 million through a civil defense program.

The document illustrates how high US-Soviet tensions were during the Cold War.

Its disclosure came as concerns flared anew about a fresh nuclear arms race between the US and Russia after US President-elect Donald Trump called for significantly bolstering the country's nuclear capabilities.

"The United States must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes," Trump said in a Twitter posting last Thursday. It was unclear what made him take on the issue, but he met with a group of Pentagon officials a day earlier.

The remark sparked concern about a renewed arms race as Russian President Vladimir Putin also called earlier in the day for bolstering "strategic nuclear forces," especially those capable of penetrating "existing and future missile defense systems."

Trump did not back down, saying a day later: "Let it be an arms race. We will outmatch them at every pass and outlast them all." (Yonhap)