A high-ranking North Korean defector has told a congressional hearing that a pre-emptive US military strike on the country would trigger automatic retaliation, with the North unleashing artillery and short-range missile fire on South Korea.

The testimony from Thae Yong-ho, former deputy chief of mission at the North Korean Embassy in London, underscored the high risk in using military force against North Korea. The Trump administration has said this is among its options in stopping leader Kim Jong-un from perfecting a nuclear-tipped missile that could strike the United States.

Thae, the highest-level North Korean defector in two decades, appeared to confirm what has long been suspected but rarely articulated by US officials - that even a selective American strike could rain a potentially devastating North Korean military response on the South Korean capital and its environs, about 25 miles (40 kilometres) south of the heavily militarised frontier.

​Thae, who is making his first visit to Washington since his defection last year, said the US and allied South Korea would win a war after a preventive military strike on the North, but there would be a “human sacrifice” inflicted on the South from the “tens of thousands” of artillery guns and short-range missiles the North has at the frontier.

“North Korean officers are trained to press their button without any further instructions from the general command if anything happens on their side,” Thae told the House Foreign Affairs Committee, referring to a US bombing or military strike. “We have to remember that tens of millions of South Korean population are living 70 to 80 kilometres away from this military demarcation line.”

He urged Washington to use “soft power” instead - enforcing sanctions and disseminating information challenging North Korean propaganda to turn people against Kim's authoritarian government. He also contended that if China allowed an “exit route” for North Korean defectors without fear of forcible repatriation there would be a “massive exodus” to the South that would cause the collapse of the North Korean system.

North Korea has called Thae “human scum” and accused him of embezzling government money and committing other crimes.

​Thae's comments come ahead of President Donald Trump's five-nation trip to Asia that will include a stop in South Korea. The US administration says it seeks a diplomatic solution to the nuclear standoff but “all options” are on the table. Trump has threatened the total destruction of North Korea if the US is forced to defend itself or its allies.

A Congressional Research Service report published last week said that conservative estimates anticipate that in the first hours of a conflict, North Korean artillery situated along the frontier could cause tens of thousands of casualties in South Korea, where at least 100,000 and possibly as many as 500,000 Americans live - including nearly 30,000 US troops. It said a protracted conflict, particularly one in which North Korea uses its nuclear, biological or chemical weapons, could cause enormous casualties on a greater scale.

Some analysts contend the risk of Kim acquiring a nuclear weapon capable of targeting the US homeland is greater than the risks associated with the outbreak of a regional war, the report said.

Democratic Senator Tammy Duckworth of Illinois wrote to Trump on Wednesday calling for him to provide the American public with declassified estimates of potential casualties, costs and outcomes from a limited or full-scale war with North Korea.

“I fear the country is being deprived of an accurate assessment of what war against the DPRK would entail,” wrote Duckworth, referring to the North's formal name, the Democratic People's Republic of Korea. Duckworth, a former US Army helicopter pilot, suffered severe combat wounds in Iraq.

In a separate development, the Senate Banking Committee announced late Wednesday it will take up legislation next week to strengthen and expand US financial sanctions against North Korea.

Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Show all 6 1 /6 Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Photos released by North Korea show Kim Jong-un talking to subordinates next to a device thought to be the new thermonuclear weapon. There is no way of independently verifying the pictures STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korea claims it has successfully tested an advanced hydrogen bomb which could be loaded onto an intercontinental ballistic missile AFP/Getty Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A diagram on the wall behind Mr Kim shows a bomb mounted inside a cone STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korean leader Kim Jong-Un (C) attending a photo session with participants of the fourth conference of active secretaries of primary organisations of the youth league of the Korean People's Army (KPA) in Pyongyang STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters

The bill targets banks and other financial institutions that continue to do business with Pyongyang and anyone else that evades existing sanctions to support with the rogue nation. The legislation also permits states and local governments to divest from or prohibit investments in companies that maintain ties to North Korea.

Congressional oversight of North Korea sanctions also would be strengthened, according to the bill, and the Treasury Department would be given a bigger role in fighting human trafficking.