South Korea detected that North Korea is continuing to prepare for a possible intercontinental ballistic missile launch, a move that would further raise tensions a day after it conducted its sixth and most powerful nuclear detonation.

Chang Kyung-soo, acting chief of the defense ministry's policy planning office, told lawmakers on Monday that North Korea was making preparations for a missile firing, but didn't give a timeframe for a potential launch. The yen extended gains against the dollar after the news.

South Korea's spy agency said there is a chance North Korea could fire an ICBM into the Pacific Ocean, adding that the isolated state was able to conduct a nuclear test at any time, the Yonhap news agency reported. North Korea had threatened last month to launch missiles toward Guam, which prompted warnings of retaliation from American military officials.

South Korea earlier in the day paved the way for the full deployment of a U.S. missile defense system while its military conducted a live-fire drill with North Korea's test site as the virtual target. The Environment Ministry on Monday conditionally approved an environmental impact report on the Terminal High-Altitude Area Defense system.

That removes the final administrative hurdle for complete installment of the missile shield, known as Thaad, which China sees as a threat to the region's “strategic equilibrium.” South Korea's Defense Ministry said it would install the system's remaining launchers "soon."

The governments in Seoul and Washington were also discussing deployment of a U.S. carrier group and strategic bombers, Yonhap said.

Following the nuclear test, President Donald Trump threatened to increase economic sanctions and halt trade with any nation doing business with North Korea -- a threat he has used before without following through. That list would include China, the U.S.'s biggest trading partner, which accounted for about a sixth of its overseas commerce.

China hit back at Trump's threat, with Foreign Ministry spokesman Geng Shuang saying the comments were "neither objective nor fair."

"What is definitely unacceptable to us is a situation in which on the one hand we work to resolve this issue peacefully but on the other hand our own interests are subject to sanctions and jeopardized," Geng said at a regular briefing in Beijing, according to the Associated Press.

Asian stocks fell on Monday as investors turned to haven assets, sending the yen, gold and Treasury futures higher. The biggest declines were in Tokyo and Seoul, with more moderate reactions elsewhere in the region.

Trump, who threatened over the weekend to pull out of the U.S.-South Korea trade agreement, took a shot at President Moon Jae-in's administration after the nuclear test. On Twitter, he said that South Korea is finding that its “talk of appeasement with North Korea will not work.”

In response, Moon's office said that war shouldn't be repeated and that South Korea and its allies “will pursue the denuclearization of the Korean peninsula through peace.” The two leaders haven't spoken since North Korea detonated what it called a hydrogen bomb.

Moon took power in May pledging to seek peace talks with Kim's regime. He initially opposed the early deployment of Thaad, though has shifted in recent months as North Korea advanced its push for an intercontinental ballistic missile that could strike the U.S.

The disagreement between allies comes as Trump's administration looks to convince China and Russia to support stronger sanctions against North Korea at United Nations Security Council. The U.S. and other nations called for an emergency meeting at 10 a.m. on Monday in New York.

While Trump didn't rule out an attack on North Korea when asked by a reporter, the focus of his Tweets and remarks by Treasury Secretary Steven Mnuchin were on tighter sanctions, not military action. China and Russia oppose military force to deal with North Korea's nuclear program.

In a call with Japanese Prime Minister Shinzo Abe, Trump “reaffirmed the commitment of the United States to defending our homeland, territories, and allies using the full range of diplomatic, conventional, and nuclear capabilities at our disposal,” the White House said in a statement.

Separately, Abe spoke with Russian President Vladimir Putin late Sunday, telling him that another strong UN resolution was needed. Abe also told South Korean President Moon that the North Korea threat was grave and imminent, Japan's Kyodo News reported. Moon hasn't spoken with Trump since the nuclear detonation.

Sunday's test, North Korea's first since Trump took office, was a “perfect success” and confirmed the precision and technology of the bomb, according to the Korean Central News Agency. Energy from the underground explosion, near the Punggye-ri nuclear test site in northeast North Korea, was about six times stronger than the nuclear test of a year ago, South Korea's weather agency said.

On Monday, South Korea's military accurately fired a surface-to-surface ballistic missile and air-to-ground rocket onto a target in the East Sea (also known as the Sea of Japan), according to a text message from the Joint Chiefs of Staff. The real-distance shooting exercise simulated the Punggye-ri site as the origin of the provocation, the joint chiefs said.

Hans Kristensen, director of the Nuclear Information Project at the Federation of American Scientists, said the international community needs to “come up with a very different game plan than just sanctions and bomber exercises.”

“Right now, everything North Korea does appears to have to be bigger and scarier,” he said by email. “The signal they're sending is the same as always: we don't care what you say, we can do this and will continue to do so until you stop threatening us. And they seem to have the resources needed to do so despite sanctions.”