China and Russia pose the biggest risks to the United States, and are more aligned than they have been in decades as they target the 2020 presidential election and American institutions to expand their global reach, US intelligence officials told senators on Tuesday.

The spy chiefs broke with President Donald Trump in their assessments of the threats posed by North Korea, Iran and Syria. But they outlined a clear and imminent danger from China, whose practices in trade and technology anger the US president.

While China and Russia strengthen their alliance, Director of National Intelligence Dan Coats said some American allies were pulling away from Washington in reaction to changing US policies on security and trade.

The directors of the CIA, FBI, National Security Agency and other intelligence agencies flanked Coats at the Senate Intelligence Committee hearing. They described an array of economic, military and intelligence threats, from highly organised efforts by China to scattered disruptions by terrorists, hacktivists and transnational criminals.

"China, Russia, Iran, and North Korea increasingly use cyber operations to threaten both minds and machines in an expanding number of ways - to steal information, to influence our citizens, or to disrupt critical infrastructure," Coats said.

"Moscow's relationship with Beijing is closer than it's been in many decades," he told the panel.

The intelligence officials said they had protected the 2018 US congressional elections from outside interference, but expected renewed and likely more sophisticated attacks on the 2020 presidential contest.

US adversaries will "use online influence operations to try to weaken democratic institutions, undermine alliances and partnerships, and shape policy outcomes," Coats said.

North Korea, ISIL remain threats

The intelligence chiefs' assessments broke with some past assertions by Trump, including the threat posed by Russia to US elections and democratic institutions, the threat the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) poses in Syria, and North Korea's commitment to denuclearise.

Coats said North Korea was unlikely to give up its nuclear weapons. Trump asserted after the Singapore summit that North Korea no longer posed a nuclear threat.

"The capabilities and threat that existed a year ago are still there," said Robert Ashley, the director of the Defense Intelligence Agency.

Plans for a follow-up Trump-Kim summit are in the works, but no agenda, venue or date has been announced.

Coats also said ISIL would continue to pursue attacks from Syria, as well as Iraq, against regional and Western adversaries, including the US. Trump, who plans to withdraw US troops from Syria, has said the armed group was defeated.

The intelligence officials also said Iran was not developing nuclear weapons in violation of the 2015 nuclear agreement, even though Tehran has threatened to reverse some commitments after Trump pulled out of the deal.

The intelligence assessment of Afghanistan, more than 17 years into a conflict that began after the 9/11 attacks on the US projected a continued military stalemate. Without mentioning prospects for a peace deal, which appear to have improved only in recent days, the report said, "neither the Afghan government nor the Taliban will be able to gain a strategic military advantage in the Afghan war in the coming year" if the US maintained its current levels of support. Trump has ordered a partial pullback of US forces this year, although no firm plan is in place.

Senators expressed deep concern about the current threats.

"Increased cooperation between Russia and China - for a generation that hasn't been the case - that could be a very big deal on the horizon in terms of the United States," said Senator Angus King, an independent who caucuses with Democrats.

Biggest counterintelligence threat

The officials painted a multifaceted picture of the threat posed by China, as they were questioned repeatedly by senators about the No 2 world economy's business practices as well as its growing international influence.

"The Chinese counterintelligence threat is more deep, more diverse, more vexing, more challenging, more comprehensive and more concerning than any counterintelligence threat I can think of," FBI Director Christopher Wray said.

He said almost all the economic espionage cases in the FBI's 56 field offices "lead back to China".

Coats said intelligence officials have been travelling around the US and meeting corporate executives to discuss espionage threats from China.

He said China has had a meteoric rise in the past decade, adding, "A lot of that was achieved by stealing information from our companies."

Tuesday's testimony came just a day after the US announced criminal charges against China's Huawei Technologies Co Ltd, escalating a fight with the world's biggest telecommunications equipment maker and coming days before trade talks between Washington and Beijing.

Coats also said Russia's social media efforts would continue to focus on aggravating social and racial tensions, undermining trust in authorities and criticising politicians perceived to be anti-Russia.

Senator Mark Warner, the panel's top Democrat, said he was particularly concerned about Russia's use of social media "to amplify divisions in our society and to influence our democratic processes" and the threat from China in the technology arena.

The Senate Intelligence Committee is one of several congressional panels, along with Special Counsel Robert Mueller, investigating whether there were any connections between Trump's 2016 and Russian efforts to influence the election.

Russia denies attempting to influence US elections, while Trump has denied his campaign cooperated with Moscow, repeatedly calling the Mueller investigation a "witch-hunt".

Coats declined to respond when Democratic Senator Ron Wyden asked whether Trump's not releasing records of his discussions with Russian President Vladimir Putin put US intelligence agencies at a disadvantage.

"To me from an intelligence perspective, it's just Intel 101 that it would help our country to know what Vladimir Putin discussed with Donald Trump," Wyden said.

The chiefs made no mention of a crisis at the US-Mexican border for which Trump has considered declaring a national emergency. Trump declared there was a humanitarian crisis at the border.