Trump then used the meeting to accuse China of meddling in the November midterms, saying it was pushing back against Trump's tariffs that he has placed on goods from China entering the U.S.
With the elections less than two months away, USA intelligence officials have said they are not seeing the intensity of Russian intervention registered in 2016 but are particularly concerned about activity by Russia, China, Iran and North Korea.
Trump did not provide evidence in the speech for his claim that China is meddling in the USA election, but it followed an ad buy in a major US paper located in a top agricultural hub by Chinese state media that pushed an anti-tariff line.
China will cut import tariffs on goods including machinery, paper, textiles and construction materials from Nov 1, in a move that would lower costs for consumers and companies as a trade war with the United States deepens.
Asked later what evidence he had, Trump said there was "plenty" but didn't immediately provide details, suggesting that some of the material was classified.
Trump condemned the ongoing Syria conflict, saying that the "Syrian regime's butchery" had been "enabled by Russian Federation and Iran".
"China has, all along followed the principle of non-interference in other countries domestic affairs". China still has a higher average tariff rate than many developed economies.
China and the United States have been in a battle over tariffs since his election.
President Donald Trump poured scorn on the "ideology of globalism" and heaped praise on his own administration's achievements in a speech to the U.N. General Assembly that drew headshakes and even mocking laughter from his audience of fellow world leaders.
President Trump simply asserted that "we have evidence" but "I can't tell you now".
China's Foreign Minister Wang Yi called Mr Trump's claim that China was attempting inference complete "nonsense".
The US president has been in an escalating trade dispute with China in recent months. Last week, in another tweet, Mr. Trump said China was "actively trying to impact and change our election by attacking our farmers, ranchers and industrial workers due to their loyalty to me".
Chinese state media has accused Washington of using trade to suppress the country's development. The American president warned world leaders at the United Nations that "all U.S. nuclear-related sanctions will be in full force by early November", adding that countries who didn't comply would face "severe consequences".
Trump said the Chinese are "trying to convince people to go against Donald Trump" because he has challenged China's trade policies that he said have resulted in Beijing taking $500 billion a year from the United States to "rebuild their country".
In August this year, Trump also tweeted that China hacked Hillary Clintons private email server.
Trump added: "I don't like it when they attack our farmers and I don't like it when they put out false messages".
He's also threatening to place tariffs on cars imported from Canada as trade talks between the two neighbours drag on. "I've noticed that other nations have also incurred certain diplomatic wrath out of Beijing for sailing their ships through". According to a senior administration official, what Trump considers election interference includes Chinese retaliatory tariffs against Midwest farmers, intimidation of journalists and academics, and funding for state-controlled news outlets.