YANJI, China - As the Korean-Chinese congregation assembled at the huge church in Yanji, the usual group of ragged orphans from North Korea appeared, begging for help.

However, mayhem suddenly erupted at the Sunday service last month in the capital of the Yanbian Korean Autonomous Region of Jilin province.

Plain-clothes North Korean agents raided the church and seized the stunted children, shouting and struggling.

Witnesses said they were led away and almost certainly taken across the border back to North Korea to face indoctrination in labour camps.

The children are a handful of those caught up in a secret war being waged on both sides of the Tumen River between militant Christian missionaries and North Korea's security service.

As a starving and despairing population loses faith in the cult of former leader Kim Il-sung, many North Koreans are joining underground churches, suspected by the regime of being at the heart of a growing resistance movement.

This year there have been reports of a large-scale manhunt in North Korea and the execution and imprisonment of North Korean Christians and their families, many of whom are fresh converts.

News of the growth of the underground church movement and its persecution leak out in letters smuggled into China by refugees passed along a network of safe houses.

The news dribbling out the country and often recorded on small scraps of handwritten letters speak of an escalated crackdown by Kim Jong-il, who assumed the country's leadership after the death of his father, Kim Il-sung, in 1994.

"Kim Jong-il is now using the army to operate house-to-house searches for Christians. They look for any pieces of paper, " said one source.

"Whoever has a Bible in their hands is accused of being a spy - anything connected with the outside world can mean arrest and death," said another.

One of the missionary groups, called Open Doors, founded 45 years ago by Brother Andrew, the Dutch-born author of God's Smuggler, claims that since 1994 Christian missionaries have established 540 underground cells and have smuggled in hundreds of thousands of Bibles.

Terry Madison, the American leader of Open Doors, based in California, claims on the group's Web site there are at least 500,000 Christians inside North Korea. They meet in secret house churches and sometimes as many as 80 people come together in caves in rural areas.

Evangelists have been seeking to convert some of the estimated 300,000 North Koreans who cross the Tumen and Yalu rivers into China each year seeking food and shelter.

The North Korean security apparatus has now taken the counter-attack into China. This year the number of refugees crossing the border has fallen sharply.

"Some think they are fewer coming over because they are toughening controls - it is certainly not because their lives are any better," said Erica Kang of the Good Friends, a Seoul-based Buddhist charity group that has carried out surveys among refugees.

Although South Korea reports there is an economic recovery under way in the North, the World Food Programme reports that a severe drought is damaging the economy and this year's harvest will be poorer than last year's.

Fear may be one reason that North Koreans stay at home. Another is that China is said to be hunting down the refugees and returning them in larger numbers. It is also believed that Beijing is tolerating the activities of perhaps more than 100,000 North Korean agents who are allegedly kidnapping refugees and murdering missionaries on the mainland.

On April 17, a 58-year-old South Korean missionary, Yi Song-chul, disappeared and was believed to have been kidnapped while living in Yanji.

In March, two South Korean missionaries, a married couple who ran a restaurant in the border town of Hunchun, were found murdered in their home. Dead alongside them were four North Korean refugees they were sheltering.

Some of those seized, such as Pastor Kim Dong-sik, who disappeared last year, are rumoured to have been taken back to North Korea and held while negotiations over a ransom take place.

For North Koreans caught in underground churches, punishment is swift and brutal. In December, a small group of Christians in Chongjin, in the country's northeast, were discovered at a meeting and arrested. The 11 men were beheaded at a public execution as a lesson to others and the women and children were sent to labour camps.

Last year the US State Department estimated that 400 Christians were executed in 1999.

Most of those who die are believed to be new converts, inexperienced at hiding their beliefs and determined to spread the Gospel. To protect themselves, the Christians are organised in small cells so that under torture they cannot reveal the names of others.

Yet letters smuggled out of the country are dangerously frank.

"We almost starved to death but you sent food unexpectedly. We have unspeakable joy," reads one letter.

"We don't know how long this suffering will go on. We have joy in our hearts. Almighty God prepared paradise in heaven for us and this mortal life is short. We are diligently preaching the Gospel. We tell people the food comes from Christians around the world. Our members are increasing day by day."

Before Korea was divided, the centre of Christianity was in the north and Pyongyang was known as the Jerusalem of Korea. There were 13,000 churches but during the 1950-53 war, hundreds of thousands of Christians fled to the South. There they established themselves as a key political force, and a centre of opposition to the dictatorship of Park Chung-hee.

The communists arrested and killed many of those who remained. A few continued worshipping and praying in secret, sometimes keeping their beliefs hidden even from their children for fear of being betrayed. During the Cultural Revolution, they were joined by some Korean Christians living in China who felt North Korea was safer.

In the latest twist, the Sunshine reconciliation policy of President Kim Dae-jung has split the South Korean Christians into conflicting camps.

One camp advocates co-operation with the Government of Kim Jong-il and the official Korean Christian Federation, which claims membership of 12,000, three functioning churches and a number of house churches.

"This is a sensitive issue," said Professor Park Kyung-seo, the human rights ambassador for the South Korean Government and a former secretary of the World Council of Churches.

"All ecumenical groups now support contact with the official church in North Korea," he said. "I believe that after there is a peaceful settlement, it will be the right time to make converts, not now."

Professor Park claimed North Korea had been printing Bibles for the past five years, doing away with the need to smuggle them in.

Kim Dae-jung's Sunshine Policy has allowed greater contact between the Koreas and prompted the North to ease its domestic propaganda against the South.

But many observers say Kim Jong-il has simultaneously stepped up internal repression.

"The human rights situation is getting worse and worse. There is much more surveillance," said one frequent visitor to North Korea. "They are putting more people in jail, especially from the educated and ruling groups, to stop defections."

Letters smuggled across the border suggest that North Koreans' faith in the Pyongyang regime has been undermined by evidence that they have been fed a false picture of South Korea - always portrayed as far poorer, more backward and oppressed than the North. As they become aware of the truth, many are ready to embrace a new set of beliefs and new purpose.