Pope Francis will seriously consider the possibility of an unprecedented visit to North Korea but some conditions will have to be met, a senior Vatican official said.

Such a trip would be a landmark in a nation known for severe restrictions on religious practice. It would be the first by a pope to the reclusive East Asian state, which does not allow priests to be permanently stationed there.

South Korean President Moon Jae-in relayed a verbal invitation from North Korean leader Kim Jong Un to the pope during a 35-minute meeting in the Vatican on Thursday.

“The pope expressed his willingness. We have to wait for it (the invitation) to be formalized,” Vatican Secretary of State Cardinal Pietro Parolin, the Holy See’s second-ranking official, told reporters on Thursday evening.

Asked if there were conditions that the North would have to meet, Parolin, speaking on the sidelines of a book presentation, said: “This will come later, once we start thinking in earnest about the possibility of making this trip, then we will have to think about conditions in which the trip can take place.

“(The pope) is willing to make the trip but a trip of this kind will need serious preparation,” added Parolin, who met separately with Moon after the South Korean president held talks with the pope.

North Korea’s constitution guarantees freedom of religion as long as it does not undermine the state. But beyond a handful of state-controlled places of worship - including a Catholic church in the capital Pyongyang - no open religious activity is allowed and the authorities have repeatedly jailed foreign missionaries.

There is little information on how many of North Korea’s citizens are Catholic, or how they practice their faith.

Kim told Moon, a Catholic, of his wish to meet the pontiff during a meeting last month and the South Korean leader announced before his trip to the Vatican that he would be relaying a message..

The pope, who is due to visit nearby Japan next year, told Moon he would “definitely answer” an invitation from Kim if it arrives, according to Moon’s office.

A meeting with Pope Francis would be the latest in a string of major diplomatic encounters for Kim this year.

The two Koreas have held three summits this year. Kim also held an unprecedented summit with U.S. President Donald Trump in Singapore in June, where the leaders promised to work toward decentralization of the Korean peninsula.