US President Donald Trump has lashed out at China for undermining its work with North Korea, as criticism over progress on denuclearisation mounts.
In a series of tweets he also said he saw no reason to resume the joint war games with South Korea that have angered North Korea.
Days ago his own defence secretary said military exercises might continue.
China has accused Mr Trump of "shifting blame" in his comments on its relations with North Korea.
A summit between Mr Trump and North Korean leader Kim Jong-un in June ended with a pledge from the North to work towards "the complete denuclearisation of the Korean peninsula". Soon after Mr Trump announced there was "no longer a nuclear threat from North Korea".
But since then many observers say North Korea is not moving fast enough to dismantle its nuclear or rocket sites.
Mr Trump's tweets appear to place the blame for these challenges squarely on China, but he also goes on to praise his personal ties with the leaders of both North Korea and China.
The confusing blend of criticism, praise and veiled threat comes as Washington faces mounting pressure to deliver results following those unprecedented talks.
Just days ago, Washington called off a trip to North Korea by US Secretary of State Mike Pompeo with the president arguing that insufficient progress had been made in dismantling the North's nuclear programme.
Why take aim at China?
In his latest comments on Twitter, Mr Trump says North Korea was "under tremendous pressure from China because of our major trade disputes with the Chinese government".
Beijing is Pyongyang's only significant ally and is thought to have significant influence over its decisions. Beijing is also Washington's most powerful long-term strategic rival in the region.
The US and China are locked in an increasingly tense trade battle and each side has imposed tariffs on one another's goods.
STATEMENT FROM THE WHITE HOUSE— Donald J. Trump (@realDonaldTrump) August 29, 2018
President Donald J. Trump feels strongly that North Korea is under tremendous pressure from China because of our major trade disputes with the Chinese Government. At the same time, we also know that China is providing North Korea with...
Mr Trump has complained about the size of the US trade deficit with China and what Washington sees as other unfair trade practices.
Despite his strong words, Mr Trump's latest tweet concluded with some optimism, saying the trade issue could be resolved by himself and China's "great President Xi Jinping".
China has attacked Mr Trump's remarks as "irresponsible" and difficult to understand.
"To solve the problem, it should look at itself instead of shifting blame," foreign ministry spokeswoman Hua Chunying said.
What progress has been made on denuclearisation?
Since the June summit, North Korea has halted its missile tests, claimed to have dismantled a nuclear testing site and returned remains that are believed to be of US soldiers killed in the 1950-53 Korean War.
But Mr Trump has called out Pyongyang for what he said was insufficient progress on their broad agreement on denuclearisation.
A recent report by Vox suggests that Pyongyang is reluctant to proceed because Mr Trump failed to live up to his alleged promise to Mr Kim that he would sign an official declaration to end the Korean War.
After a UN report found that North Korea was continuing to work on its nuclear programme, the US urged the international community to maintain sanctions and economic pressure on Pyongyang.
In his latest series of tweets, Mr Trump accused Beijing of providing North Korea with "considerable aid", suggesting China was softening the blow of sanctions.
"This is not helpful!" Mr Trump said.
Will war games resume?
The joint military exercises between Washington and Seoul have long angered Pyongyang.
Following the summit in June, Mr Trump made what has widely been seen as a concession to Kim Jong-un and agreed to cancel the drills.
In his latest tweets, US president insisted his personal relationship with Mr Kim remained "a very good and warm one" and that there was no reason to restart the "war games" with the South.
But he added that if they did restart, the exercises would be "bigger than ever".
Mr Trump's tweets appear to contradict his defence secretary, who on Tuesday said the suspension of several joint exercises was a "good faith measure" and that "we have no plans to suspend any more."
Since the end of the Korean war, when Washington fought alongside Seoul against the North, the US has had troops stationed in South Korea.
About 29,000 US soldiers are based in the South, under a security agreement reached after the war ended in 1953.