The Vatican's foreign minister Paul Richard Gallagher began an unprecedented six-day visit to Communist-ruled Vietnam on Tuesday as the two sides work on closer relations.

Gallagher's trip, the first to Vietnam for a Vatican foreign minister, could be followed by other top-level visits, including Pope Francis, who the Vatican has said is planning a trip to Indonesia.

Although no time frame for that trip has been announced, diplomats expect that could be combined with other countries in Southeast Asia, including Vietnam, if Francis' health allows. Gallagher said in January the pontiff was keen to visit Vietnam.

Vietnam's foreign ministry said Gallagher will meet Vietnamese counterpart Bui Thanh Son late in the afternoon on Tuesday.

Vietnam News Agency reported Gallagher would also meet Prime Minister Pham Minh Chinh and will hold mass at cathedrals in the capital city Hanoi, in Hue, in the centre of the country, and in Ho Chi Minh City, Vietnam's southern business hub.

The country of 100 million people is home to about 7 million Catholics, government estimates showed, making it one of the largest communities in East Asia.

The Vatican and Vietnam broke relations after the Communist Party took over the reunited country at the end of the Vietnam War in 1975. At the time, the authorities viewed the Catholic Church in Vietnam as having been too close historically to former colonial power France.

Vietnam's government places certain restrictions on Catholic activities, such as the number of parishes, according to UCA, an independent Catholic news agency that specialises in Asia.

In July, during a visit by Vietnam's then President Vo Van Thuong to Pope Francis, the pair agreed to Archbishop Marek Zalewski becoming Resident Papal Representative in Hanoi, another unprecedented step that had been years in the making.