"We will say, however, that we are confident in the legality of the actions we announced and carried out last week", a senior State Department official said.
The U.S.is circulating a draft resolution at the United Nations that would bar crude oil shipments to North Korea, ban the nation's exports of textiles and prohibit employment of its guest workers by other countries, according to a diplomat at the world body. Sunday's test of what North Korea said was an advanced hydrogen bomb was its largest by far. Han Tae Song, the North's ambassador to the United Nations in Geneva, noted Tuesday that more North Korean missiles and nuclear tests are planned. They also say the health rumors that have spurred local concerns about the system's powerful radar component are groundless and no such issues have been reported at other THAAD sites.
The ministry of national defence, meanwhile, said it was preparing for joint measures with the United States military for a show of force against the North Korean provocation.
Russian President Vladimir Putin has called for talks with North Korea, saying sanctions are not a solution.
Resolving the North Korean nuclear crisis is impossible with sanctions and pressure alone, Russian President Vladimir Putin said after meeting his South Korean counterpart on Wednesday, again urging dialogue to achieve progress.
But Putin was reluctant to support Moon's push for harsher sanctions.
The possibility of increased USA tariffs comes at the same time the Moon administration is facing Chinese criticism for deploying the American THAAD missile defense system.
He pointed to the Iran nuclear deal, which was negotiated successfully by the Obama administration but has looked fragile under Trump, saying "if the USA had spent even one fifth of the time and effort on the North Korean issue compared with the Iranian case, then the North Korean nuclear issue could have been resolved".
The announcement dramatically upped the stakes in its standoff with the global community over its banned weapons programmes, which have seen it subjected to seven sets of UN Security Council sanctions so far.
Washington has denied this, claiming that the system's sole goal is to fend off aggression from Pyongyang.
Pyongyang also conducted its sixth nuclear test on Sunday, prompting vehement reprimands from neighbouring Japan and the United States. Abe is due to meet Putin separately later in the day.
Speaking at an economic forum in Russia's eastern port of Vladivostok, Putin said Thursday he believes the Trump administration is "willing to resolve the situation".
In a separate phone call with German Chancellor Angela Merkel, South Korea President Moon Jae-In and Japan Prime Minister Shinzo Abe, Trump underscored the need to give a coordinated response to North Korea.