China arrested two South Korean Christian pastors for allegedly helping to smuggle North Korean defectors out of China, the latest in a string of detentions and expulsions aimed at South Korean Christians in the past several months.

One of the pastors was arrested with his wife as the pair tried to board a flight from China to South Korea and the other was arrested at a Chinese hotel, the Yonhap news agency reported, citing South Korean groups.

The pastors remained under arrest Wednesday, however, their wives had been released, Reuters reported.

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The arrests come on the heels of a mass expulsion of South Korean missionaries from the Jilin province – which borders North Korea – in just the past few months, according to media reports noted by Christianity Today. Anywhere from 30 to 70 pastors have been kicked out of the area.

“Chinese authorities raided the homes of the missionaries, citing a problem with their visas, and told them to leave,” a human rights activist told AFP.

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There are 500 registered South Korean missionaries in China, according to Christianity Today; however, the publication notes the real number may be four times as high.

Word of the arrests marked the latest flashpoint in escalating tensions between China and South Korea, as Rex Tillerson makes his first trip to Asia as Secretary of State. South Korea and Japan have supported U.S. efforts to increase diplomatic and economic pressure on North Korea -- and the countries' naval forces conducted missile defense information-sharing drills with the U.S. on Wednesday.

But Beijing has raised red flags over the drills, and has claimed the planned deployment of a missile defense system known as THAAD could peer into Chinese territory, weakening its own nuclear defenses.

Tillerson, who arrived in Tokyo late Wednesday at the start of his four-day, three-nation tour, could provide some reassurance to nervous allies. He's set to meet Thursday with Japanese Prime Minister Shinzo Abe and Foreign Minister Fumio Kishida.

After Tokyo, Tillerson visits South Korea, also caught up in political upheaval after last week's ouster of its president, Park Geun-hye, over a corruption scandal. Park had been in lockstep with Washington's efforts to isolate North Korea. The favorite to succeed her is Moon Jae-in, a moderate who wants to engage North Korea's government.

Tillerson's final leg involves meetings with several senior Chinese officials in Beijing.

The Associated Press contributed to this report.