U.S. President Donald Trump has for the first time called to order a meeting of the UN Security Council, where he accused China of attempting to interfere in the upcoming U.S. Congressional elections.

Trump, speaking in front of world leaders, did not present evidence for his claim, which came amid an ongoing special counsel investigation into Russia's attempts to interfere in the 2016 U.S. election and concerns that the November elections could also be vulnerable.

"Regrettably, we found that China has been attempting to interfere in our upcoming 2018 election," Trump said. "They do not want me or us to win because I am the first president ever to challenge China on trade."

U.S. officials did not immediately respond to requests for comment. With the elections less than two months away, U.S. 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.

China's top diplomat, Wang Yi, denied Trump's accusation.

"We did not and will not interfere in any country's domestic affairs," he told the Security Council. "We refuse to accept any unwarranted accusations against China."

'Propaganda ads'

The U.S. president later tweeted that "China is actually placing propaganda ads in the Des Moines Register and other papers, made to look like news."

"That's because we are beating them on Trade, opening markets, and the farmers will make a fortune when this is over!"

He appears to be referring to an inserted supplement in the paper that was produced by state-controlled English outlet China Daily.

After Trump's tweet, the chief political reporter from the Des Moines paper tweeted out a link to an article in that same paper, which explains that China Daily has done the same in national papers. Both the Washington Post and the Wall Street Journal have run similar paid supplements.

President Trump is tweeting about a four-page insert that appeared in the Des Moines Register over the weekend. Some context: <a href="https://t.co/66S1VqWuHY">https://t.co/66S1VqWuHY</a> <a href="https://t.co/IDIcqW9vUC">https://t.co/IDIcqW9vUC</a> —@brianneDMR

Later Wednesday, at a lengthy news conference in which he took questions on everything from his embattled Supreme Court nominee to trade with North Korea, Trump said that China "would like to see me lose an election" and that no country has ever stood up to China like his administration.

He said he can't disclose the evidence, but that it will come out, adding that his allegation did not come "out of nowhere."

A senior administration official briefed reporters Wednesday after Trump said there was "plenty of evidence" that China is interfering in U.S. midterm elections.

The official, who spoke on condition of anonymity, said Vice-President Mike Pence is expected to deliver a speech on the subject next week in Washington. The official said China is hurting farmers and workers in states and districts that voted for Trump.

More tough talk on Iran

Trump also used his moment chairing the Security Council meeting about nuclear proliferation to issue a strong warning to nuclear-aspirant Iran, which he deemed the "world's leading sponsor of terror" fuelling "conflict around the region and far beyond."

The U.S. president has withdrawn his country from the 2015 nuclear accord with Iran, accusing the country of destabilizing actions throughout the region and support for terrorist groups like Hezbollah. Tough sanctions are due to kick in against Tehran in November, and Trump warned that there would be "severe consequences" for any country that defied them.

Watch both Trump and Rouhani talk about the nuclear deal.

Iranian president says U.S. decision was a 'political mistake' 0:27

Despite his tough talk, Trump said he could envision relations with Iran moving along a similar "trajectory" as ones with North Korea. A year ago at the UN, Trump belittled its leader Kim Jong-un as "Rocket Man" and threatened to annihilate the country, but on Wednesday he touted the "the wonderful relationship" with Kim and teased that details of a second summit between the two men could be released soon.

He also condemned violence in the ongoing bloody civil war in Syria, saying that the "butchery is enabled by Russia and Iran."

Wearing an earpiece, Trump alternated between listening to simultaneous translation as other leaders and ministers spoke and scanning the room or his prepared notes.

Bolivian leader Evo Morales launched a blistering critique of U.S. policy toward Iran as well as Trump's immigration policies that resulted in the separation of migrant children from their parents.

Trump sat stone-faced during the remarks, looking straight ahead. He thanked Morales for his remarks, before recognizing the next leader.

China's Foreign Minister Wang Yi and the Chinese delegation listen as Trump address the UN Security Council meeting on Wednesday. (Carlos Barria/Reuters)

The high-profile Security Council meeting came a day after Trump poured scorn on the "ideology of globalism" and heaped praise on his own administration's achievements in a speech to the UN General Assembly that drew head shakes and even mocking laughter from his audience of fellow world leaders.

"The U.S. will not tell you how to live and work or worship," Trump said as he unapologetically promoted his "America First" agenda. "We only ask that you honour our sovereignty in return."

Speaking in triumphal terms, Trump approached his address to the world body as something of an annual report to the world on his country's progress since his inauguration.

He showcased strong economic numbers, declared that the U.S. military is "more powerful than it has ever been before" and crowed that in "less than two years, my administration has accomplished more than almost any administration in the history of our country."

With files from Reuters