Activists helping refugees to flee North Korea fear that the death of leader Kim Jong-Il will lead to a crackdown on people trying to escape repression and hunger in the communist state.

The North is believed to have stepped up patrols along its border with China, as well as security checks in public places, in an attempt to prevent defections in the wake of Kim's death on December 17.

"I can't reach most of my contacts in the North. All of their mobile phones are switched off, probably to dodge heightened surveillance by the police," said Kim Seung-Eun, a South Korean pastor who helps the refugees.

"I'm hoping nothing serious has happened to them," he told AFP.

Both China and North Korea have reinforced troops along the border, he said, citing a source near the frontier who communicated via a mobile telephone smuggled into the reclusive country from China.

About 23,000 North Koreans have fled their poverty-stricken homeland for South Korea since the 1950-1953 war, the vast majority in recent years.

They typically escape on foot via an increasingly porous border to neighbouring China, where they hide out and then travel to a third country to seek resettlement in South Korea.

Escaping across the border between the two Koreas -- the world's most heavily militarised frontier -- is almost impossible.

Activists expect the North to tighten restrictions on its people's movements during the politically delicate power transfer to Kim's young and untested son Jong-Un.

News websites run by defectors in Seoul have reported that even before Kim's death, Jong-Un was overseeing efforts by the North's military and police to strengthen border controls and clamp down on a stream of refugees.

South Korea-based Christian evangelist groups are at the vanguard of efforts to help those fleeing the North.

Activists operate secret networks of missionaries in northeast China to try to help the escapees secretly travel along what is called the "underground railroad" after the US network which helped escaped slaves.

China considers such North Koreans as economic migrants rather than political refugees and repatriates them, despite criticism from rights groups that they face harsh punishment back home.

The Seoul-based activist group Citizens' Alliance for North Korean Human Rights estimates that Beijing repatriates about 5,000 North Korean refugees each year.

Beijing has for years propped up the regime in Pyongyang, fearful that a collapse would lead to a wave of refugees flooding into its territory.

Jo Seong-Rae of Pax Koreana, a church-based activist group, said his contacts in China and the North were "lying low" on heightened alert, waiting to see whether new steps would be taken to deal with refugees.

"It will probably become a lot tougher for North Koreans to get out of the country for a while, with security at the border and within the country being strengthened during this tense period," he said.

Beijing's apparent efforts to preserve close relations with the smaller but nuclear-armed neighbour will lead to tougher action against the estimated 100,000 North Korean refugees in China, he said.

"I'm afraid defectors hiding in China will face an increased danger of capture from now on, with Beijing trying to boost ties with Jong-Un's regime and prevent instability on the border," Jo said.

Reverend Chun Ki-Won of Durihana, a Seoul-based group that helps arrange defections from the North, said his missionaries in the Chinese cities of Dandong and Yanji had gone into hiding to evade arrest.

"A Chinese crackdown on our missionaries helping North Koreans has already intensified, which means the situation for the defectors could get worse," he said.