The report in state media, quoting an earlier military statement, came hours after President Donald Trump threatened North Korea with "fire and fury".
The North's official news agency said it was considering a plan to fire medium-to-long-range rockets at Guam, where US strategic bombers are based.
The exchanges mark a sharp rise in rhetoric between the two countries.
The UN recently approved further economic sanctions on North Korea, which Pyongyang said were a "violent violation of our sovereignty", warning the US would "pay a price".
'Met with fire and fury'
On Wednesday, the official KCNA news agency said North Korea was "carefully examining the operational plan for making an enveloping fire at the areas around Guam" using its domestically made medium-to-long-range Hwasong-12 missiles.
The news agency reported a military statement issued on Tuesday, which probably came in response to US military drills in Guam.
In a message to the public, the governor of Guam Eddie Baza Calvo said there was currently "no threat" to the island and the Marianas archipelago, but that Guam was "prepared for any eventuality".
North Korea's statement is the latest stage in a heating up of rhetoric and tension.
Pyongyang, which has tested nuclear devices five times, launched two intercontinental ballistic missiles (ICBM) in July, claiming it now had the ability to hit the mainland US.
On Tuesday, media reports in the US claimed the North had achieved its goal of making a nuclear warhead small enough to fit inside its missiles.
While not confirmed, this was seen as one of the last obstacles to North Korea being a fully nuclear armed state.
A report in the Washington Post, citing US intelligence officials, suggested North Korea is developing nuclear weapons capable of hitting the US at a much faster rate than expected.
A Japanese government defence white paper also said the weapons programme had "advanced considerably" and that North Korea possibly now had nuclear weapons.
In response, President Trump warned North Korea to stop threatening the US, saying they would be "met with fire and fury like the world has never seen".
However veteran US Senator John McCain was sceptical about Mr Trump's statement, saying he was "not sure that President Trump is ready to act".
'Scary' situation - BBC's Yogita Limaye in Seoul, South Korea
On the streets of Seoul, barely 50km (30 miles) from the border with North Korea, the latest developments have drawn mixed reactions. Kim Seong-su, 62, said he thought Pyongyang was bluffing to preserve its regime and justify its nuclear programme.
But others are more concerned. Yeon Eui-sook says she finds the situation scary. "I hope everyone can live in peace. Kim Jong-un keeps doing this and making us worry," she said.
Analysts say the language from Pyongyang always gets more aggressive in August, when the US and South Korea conduct joint military exercises. But this time - with a US president who also uses strong words - the confrontation is getting even fiercer than usual.
North Korea had reacted angrily after the fresh sanctions were announced on Saturday by the UN, in an attempt to pressure it into giving up its nuclear ambitions.
The sanctions aim to reduce North Korea's export revenues by a third.
KCNA said North Korea would retaliate and make "the US pay a price" for drafting the new measures.
It called the sanctions a "violent violation of our sovereignty", the news agency said.
China, which is Pyongyang's closest ally, has said it is "100%" committed to enforcing the latest round of sanctions.
Russia and China have previously differed with others on how to handle Pyongyang, but in recent months have joined calls for North Korea to stop its missile tests - while also urging the US and South Korea to halt military drills, and withdraw an anti-missile system from the South.
- The 541 sq km (209 sq miles) volcanic and coral island in the Pacific between the Philippines and Hawaii.
- It is an "unorganised, unincorporated" US territory, with a population of about 163,000.
- US military bases cover about a quarter of the island. About 6,000 personnel are based there and there are plans to move in thousands more.
- It is a vital staging post for US operations, giving access to potential flashpoints like the South China Sea, the Koreas and the Taiwan Straits.