The Chinese tech giant has always rejected claims it represents a security risk.
But speaking to reporters on the plane to the UK, Mr Pompeo said: "Our view of Huawei has been that putting it in your system creates real risk.
He added: "This is an extension of the Chinese Communist Party with a legal requirement to hand over information to the Chinese Communist Party."
He added that "American information only should pass through trusted networks, and we’ll make sure we do that" and suggested the UK could "relook" at the decision in the future.
The Prime Minister defied the president by giving the green light to the Chinese firm despite US warnings that it could hamper intelligence-sharing with Washington and the other members of the Five Eyes alliance.
Mr Johnson, who spoke to Mr Trump on Tuesday, said the Government’s decision would not damage the “extremely valuable” security co-operation with the Five Eyes alliance which includes the US.
The decision has caused deep unease on the Tory benches, with discussion of a possible rebellion when the matter comes to the Commons, although the Prime Minister can normally rely on a comfortable majority.
The UK’s National Security Council (NSC) agreed on Tuesday to allow “high-risk vendors” to play a limited part in building the 5G network.
At a 90-minute meeting, Defence Secretary Ben Wallace argued against the move, according to The Times, but was said to have been a “lone voice”.
Senior Tory MPs including former leader Sir Iain Duncan Smith and ex-Brexit secretary David Davis are among those to express displeasure.
Mr Trump has refrained from a Twitter outburst on the decision but officials in Washington said they were "disappointed by the UK’s decision".
"There is no safe option for untrusted vendors to control any part of a 5G network," an official said.
A series of senior congressional figures spoke out to condemn the move.
Tom Cotton, a member of the Senate intelligence committee, said: "Allowing Huawei to build the UK’s 5G networks today is like allowing the KGB to build its telephone network during the Cold War."
The European Union also unveiled security guidelines for the next generation high-speed wireless networks that stop short of banning Huawei, in a further blow to the US campaign against the firm.
Mr Pompeo’s two-day visit is likely to offer the first real indication of the extent of any damage to the so-called special relationship.
The US administration has consistently argued that giving Huawei a role in 5G could allow the Chinese a "back door" into the telecoms network through which they could carry out espionage or cyber attacks.
The Government has acknowledged Huawei is not a "trusted" supplier but argues that by banning it from the most sensitive elements of the network and restricting its involvement to 35%, it can manage the risks.
The clash comes at a sensitive moment in US-UK relations, just as Mr Johnson is hoping to make rapid progress on a trade deal.
The PM appears to have concluded that honouring his general election pledge to "level up" the "left-behind" areas of the country must be the priority.
Rolling out 5G across the country is regarded as key to improving economic performance and excluding Huawei would mean delays and higher costs.