US Secretary of State Mike Pompeo revived a spat with the Vatican Wednesday during a visit to Rome, a month ahead of the US elections and hot on the heels of a diplomatic breach that experts see as an effort to win conservative Catholic votes.

Pompeo is not meeting Pope Francis because the pontiff avoids such audiences in campaign periods, a Vatican source said.

Analysts say the pope has been angered by Pompeo's public calls for a historic Vatican-China accord to be scrapped.

Instead, Pompeo spoke at a symposium organised by the Holy See's US embassy, where he renewed an appeal for the Vatican to take a stand against China, pushing a religious freedom theme used in Donald Trump's re-election campaign.

"Nowhere is religious freedom under assault more than in China," said Pompeo, an evangelical Christian.

"I call on every faith leader to find the courage to confront religious persecution," he said, insisting that for the Catholic Church "earthly considerations shouldn't discourage principled stances".

US Secretary of State Mike Pompeo will speak at a symposium organised by the Holy See's US embassy in the Italian capital
US Secretary of State Mike Pompeo will speak at a symposium organised by the Holy See's US embassy in the Italian capital POOL / ARIS MESSINIS

Pompeo had gone on the offensive earlier this month, calling a Sino-Vatican 2018 agreement on appointing bishops, which is up for renewal, a risk to the church's "moral authority" given Beijing's human rights record.

Francis has been working hard to repair ties with China and the Vatican has not appreciated Pompeo's public interference.

Archbishop Paul Gallagher, the Holy See's secretary for relations with states, told journalists at the symposium that Pompeo's statement this month had been "received critically".

"Normally when you are preparing these visits between high level officials, you negotiate the agenda of what you are going to talk about privately, confidentially. It's one of the rules of diplomacy," he said.

Secretary of State Cardinal Pietro Parolin, who is to meet Pompeo Thursday, told journalists the Vatican "knows that there is a lot of opposition, of criticism", but "we have decided to move forward" on renewing the deal.

Pompeo held talks with Italy's Prime Minister Giuseppe Conte
Pompeo held talks with Italy's Prime Minister Giuseppe Conte POOL / GUGLIELMO MANGIAPANE

Powerful cardinal Oscar Maradiaga said the US intervention was unwelcome, and clearly linked to the election campaign.

"They're looking for Donald Trump to get elected, and everything is based on that logic. In this sense, I don't think they're acting in the interests of Americans," he said in an interview with the Repubblica daily on the eve of Pompeo's visit.

Francis's opening towards China had particularly angered the "anti-Francis" network close to the US president, he said, led by Archbishop Carlo Maria Vigano, the Vatican's former envoy to the US, and Steve Bannon, Trump's former advisor.

Massimo Faggioli, a theologian at the Villanova Catholic University in the US, said there was a concerted effort under way to "turn a certain anti-Francis and anti-Vatican sentiment, which has become more visible in recent years, into votes for Trump".

"Vatican diplomacy is being used for propaganda," he said.

According to the Pew Research Centre, around half of Catholic registered voters describe themselves as Republicans or lean toward the Republican Party, while roughly the same identify with or lean toward the Democratic Party.

A "notable majority of white Catholics" voted for Trump last time, Faggioli said, and "the plan is to keep this bloc of white Catholic voters in some states where it is especially needed".

Pompeo also met Wednesday Italy's Prime Minister Giuseppe Conte and Foreign Minister Luigi Di Maio to discuss US efforts to deter European allies from using equipment by Chinese manufacturer Huawei in developing their 5G networks.

The US accuses Huawei of being a tool for Chinese espionage.

Pompeo said the US was urging Rome to "carefully consider the risk... presented by technology with ties to the Chinese communist party's surveillance state".

Italy insists its Golden Power law -- which allows the government to impose conditions, restrictions or a ban on foreign investment in strategic industries -- protects it.

Nonetheless, Conte promised last week to take stronger measures to ensure national security in the country's 5G networks.