(CNN) A day after North Korea tested a ballistic missile that it claims can reach all of the United States, US President Donald Trump ripped China for not reining in Kim Jong Un and his missile program.

"I am very disappointed in China," Trump wrote in a pair of Twitter posts. "...they do NOTHING for us with North Korea, just talk. We will no longer allow this to continue. China could easily solve this problem!"

North Korea's test Friday of an intercontinental ballistic missile was its second of the long-range weapon in a month. The first on July 4 showed the missile had the range to reach Alaska.

Friday's test was designed to show the Hwasong-14 missile's maximum range with a "large-sized heavy nuclear warhead," a statement from Pyongyang's Korean Central News Agency said.

It said Washington should regard the launch as a "grave warning."

"The whole US mainland" is now within North Korea's reach, KCNA quoted Kim as saying. The North Korean leader called Pyongyang's weapons program "a precious asset" that cannot be reversed or replaced, according to the agency.

On Sunday, North Korea said it will "respond with firm action" if the US continues to pursue sanctions against it, according to KCNA, quoting a foreign ministry spokesman.

"The US needs to stop with its delusion of trying to harm us, by clearly understanding the strategic status of the DPRK which soared up as the world's nuclear and missile power, and our military and peoples' strong will to revenge our enemies to destruction," the spokesman said.

"If the US continues to frantically cling on to the so-called 'strong sanctions' and military adventures against us, we will respond with firm action of justice that we had already made clear," the spokesman added.

Expert: Missile test puts US mainland in range

Weapons experts say if Friday's missile had been fired on a flatter, standard trajectory, it would have threatened major US cities. Los Angeles, Denver and Chicago would all be well within its range, with the possible ability to reach as far as New York and Boston, according to David Wright, a missile expert at the Union of Concerned Scientists.

However, early analysis of Friday's test cannot determine how heavy a payload the missile was carrying in its warhead,

Wright said. The heavier the payload, the shorter the range. But if North Korea's assertions about Friday's test are true, Pyongyang may be even more advanced in its missile program than previously thought.

Earlier in the week, a US official told CNN the United States believed that North Korea would be able to launch a reliable nuclear-capable intercontinental ballistic missile by early 2018.T

The official said that while North Korea can currently get a missile "off the ground," a lot of undetermined variables remain about guidance, re-entry and the ability to hit a specific target.

China to North Korea: Stop escalating tensions

China, a longtime North Korean ally, issued a statement Saturday condemning the missile launch and asked Pyongyang to "stop taking actions that would escalate tensions" on the Korean Peninsula.

"The UN Security Council has clear regulations on North Korea's launch activities that use ballistic missile technologies. China is opposed to North Korea's launch activities in violation of UN Security Council resolutions and against the will of the international community," Chinese Foreign Ministry spokesman Geng Shuang said.

Beijing's statement reiterated its long-held position on North Korea's missile program.

But Trump said in his tweets Saturday night that China has been taking advantage of the US.

He tied trade policy to the North Korea situation and took a swipe at his predecessors in the Oval Office."Our foolish past leaders have allowed them (China) to make hundreds of billions of dollars a year in trade" while allowing North Korea's missile program to become a direct threat to the US mainland, he wrote.

China earlier this year proposed that North Korea freeze its nuclear weapons and missile programs in exchange for the United States and South Korea ending a string of military exercises that North Korea considers a threat to its security.

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