The arrival of the 23rd Chemical Battalion came only hours after North Korea announced its military had been green-lighted to use nuclear weapons on U.S. targets, Stars and Stripes reported.
The U.S. Army announced in October the 250-member unit would relocate from a base in McCord, Wash., where it had been stationed for eight years.
North Korea is believed to have an extensive stockpile of biochemical weapons.
South Korea's defense chief said Thursday North Korea had moved a missile with "considerable" range to its east coast, The New York Times reported.
The missile was not considered capable of hitting the United States, but Pyongyang said Thursday its military had been authorized to "take powerful practical military counteractions" against U.S. military aircraft that recently overflew the peninsula during joint military exercises with South Korea.
The actions were the latest in a political chess game between the communist country and the South Korean-U.S. alliance.
The Pentagon said Wednesday the U.S. Army is speeding a mobile missile defense system to Guam to protect U.S. forces in the Pacific from a possible North Korean attack.
Deploying the sophisticated Terminal High Altitude Area Defense, or THAAD, system to the U.S. territory two years ahead of schedule is "a precautionary move to strengthen our regional defense posture against the North Korean regional ballistic missile threat," the Pentagon said.
The land-based missile system, designed to shoot down short-, medium- and intermediate-range ballistic missiles, is expected to be fully deployed in Guam in the next few weeks, the Pentagon said.
U.S. Defense Secretary Chuck Hagel said Pyongyang's threats had escalated to "a real and clear danger" to the United States and to its regional allies.
Hagel specifically cited "the threats that the North Koreans have leveled directly at the United States regarding our base in Guam, threatened Hawaii, threatened the West Coast of the United States."
"We have to take those threats seriously," he said.
Despite the concern, Pyongyang has not established its missiles can hit Guam or Hawaii, much less the U.S. mainland, the Los Angeles Times said. It is also not clear if North Korea has a nuclear warhead small enough to be carried on its missiles, the Times said.
North Korea's state news agency quoted an unnamed military spokesman Wednesday as saying the North's military had been cleared to attack using "smaller, lighter and diversified nuclear" weapons and Pyongyang had "ratified" plans for a "merciless" nuclear operation against the United States.
The "moment of explosion is approaching fast" and war could break out "today or tomorrow," the official Korean Central News Agency quoted the spokesman as saying.
The North announced Tuesday it would resume operations to produce weapons-grade plutonium at a formerly shuttered nuclear reactor at its Yongbyon Nuclear Scientific Research Center.
Photographs published Wednesday on the website 38 North, which follows North Korean developments, show new construction at the aging reactor, dating back several weeks.