US Secretary of State Mike Pompeo urged Britain Wednesday to be cautious about China's role in its 5G network, during a visit to London that also highlighted the old allies' differences on Iran.

Following talks with Prime Minister Theresa May and Foreign Secretary Jeremy Hunt, Pompeo also condemned ‘disgusting’ politicians who backed Venezuela's Nicolas Maduro, and urged European nations to take back captured Islamic State group fighters.

In a joint press conference with Hunt, US President Donald Trump's top diplomat said they had ‘discussed at some length the importance of secure 5G networks’.

‘The United States has an obligation to ensure that places where we will operate, places where American information is, places where we have our national security is at risk, that they operate inside trusted networks, and that's what we'll do.’

The US has banned government agencies from buying equipment from Chinese firm Huawei over fears Beijing could spy on communications and gain access to critical infrastructure.

A leak from Britain's National Security Council last month suggested the government in London is planning a limited role for Huawei in its 5G network.

But Hunt insisted no decision had been taken, adding that Britain would ‘never take a decision that compromised our ability to share intelligence’ with its close allies.

Pompeo urged Britain to be ‘vigilant and vocal against a host of Chinese activities that undermine the sovereignty of all nations’.

Noting the Belt and Road Initiative, Beijing's global infrastructure project, he said: ‘China peddles corrupt infrastructure deals in exchange for political influence.

‘Its bribe-fuelled debt trap diplomacy undermines good governance and threatens to upend the free market economic model on which so many countries depend.’

- 'Disgusting' Venezuela support -

Their talks also covered Iran, which said Wednesday it had stopped respecting limits on its nuclear activities agreed under a 2015 deal with major powers until they find a way to bypass renewed US sanctions.

The US pulled out of the deal last year, but other signatories, including Britain, have tried to save the agreement with a trade mechanism meant to bypass reimposed US sanctions.

Pompeo said Tehran's announcement was ‘intentionally ambiguous’ and Washington would wait and see what actions it took.

He added: ‘There are provisions in the sanctions we put in place that allow humanitarian aid and certain products to get into the country.’

He added: ‘When transactions move beyond that... we will evaluate, review it, and if appropriate there will be sanctions against those who were involved in that transaction.’

Hunt said Iran's announcement was an ‘unwelcome step’ and urged the country to stick to the deal.

‘Should Iran cease to observe its nuclear commitments, there would of course be consequences,’ he said.

Elsewhere, Pompeo pressed the need for countries involved in the international coalition against the Islamic State group in Iraq and Syria to take back fighters and civilians captured or held in refugee camps there.

‘We've rounded them up, they are now detained and they need to continue to be detained so they cannot present additional risk to anyone anywhere in the world,’ he said.

The US diplomat also had tough words for supporters of Venezuelan President Maduro, following a question about British opposition Labour leader Jeremy Corbyn's criticism of outside intervention in the country.

‘It is disgusting to see leaders, not only in the United Kingdom but in the United States as well, who continue to support the murderous dictator Maduro,’ Pompeo said.

‘No leader from a country with Western democratic values ought to stand behind them.’

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