The Vatican's de facto foreign minister arrived in Vietnam on Tuesday hoping to ease strained ties between the communist Southeast Asian nation and the Holy See, and pave the way for a potential visit by Pope Francis.

Archbishop Paul Gallagher is the most senior official to visit Vietnam since diplomatic relations were cut at the end of the war with the United States in 1975, when the communist government expelled the apostolic delegate.

In recent years the two have tried to restart diplomatic relations, and Gallagher's trip — which will include tours of Hanoi, southern business hub Ho Chi Minh City and ancient capital Hue — comes after a Vietnamese delegation visited the pope earlier this year.

Vietnam has around six million Catholics, who make up roughly six percent of its population.

The government last year invited Pope Francis to visit the country and Gallagher is in Hanoi to work out the details.

Gallagher has said the Pope is "keen to go" but no date has been set.

He met Vietnamese counterpart Bui Thanh Son in the capital city and is scheduled to meet Vietnamese Prime Minister Pham Minh Chinh on Wednesday.

In his meeting with Son, Gallagher said he wanted to "express gratitude and thankfulness for the progress that has been made in relations between Vietnam and the Holy See in the last year".

Gallagher said previously that the Holy See was hoping to encourage the Vietnamese government "along the line of greater religious freedom, which they have in the constitution".

'Second-class citizens'

Rights groups say the country restricts religious freedoms, and the United States Commission on International Religious Freedom — a US government advisory body mandated by Congress — has put Vietnam on its list of "countries of particular concern".

It cites the country's "persistent — and apparently worsening — egregious, ongoing, and systematic violations".

Hanoi has rejected the allegations.

Anh Tran, associate professor of historical and systematic theology at Santa Clara University, told AFP that the communist government, in their Marxist interpretation, believes "religious freedom" is the "right to believe or not to believe".

But he said that "any social involvement or influence of religious groups... is forbidden".

Le Quoc Quan, a Catholic blogger and religious freedom activist based in the US, told AFP that "so many Vietnamese are expecting to meet Paul Gallagher" at the three masses he will hold.

But Quan said he doubted whether the visit would lead to substantive improvements for Catholics in the country.

"They (authorities) have regarded Vietnamese Catholics as second-class citizens for a long time."

Catholics have historically been perceived by the government as having close links with former colonial power France. 

Gallagher's visit comes after Vietnam and the Holy See agreed last year to a "Resident Papal Representative" for the communist nation, after a meeting between the then President Vo Van Thuong and Pope Francis.

Polish archbishop Marek Zalewski was appointed to the job last December.