North Korea has strengthened its surveillance of its people in areas bordering China to crack down on those contacting defectors in South Korea ahead of its key party congress, sources said Tuesday.

The Ministry of State Security, the North's intelligence agency, has sent a letter expressing its strong allegiance to North Korean leader Kim Jong-un, according to the sources familiar to North Korean affairs.

In the letter, the ministry pledged in March to beef up its surveillance along the border as the Workers' Party of Korea is preparing to hold its first party congress in more than three decades in May.

"The North is trying to strengthen its control over people in the border areas on the grounds that internal information in North Korea has leaked to the South Korean media," a source said.

The ministry is carrying out special operations to arrest North Koreans who contact their family members in South Korea via mobile phones, the sources added.

Defectors living in South Korea send money to their kin in the North through brokers in China or the North. They talk over the phone along the border regions where Chinese mobile phones work.

"The authorities have increased the number of agents to monitor North Koreans at public places, such as markets," the source said.

"North Korea has been beefing up its crackdown over its people.

Those who are at risk the most are North Koreans who have family members who have defected to the South."

A recent report by Amnesty International showed that ordinary North Koreans are at risk of being sent to political prison camps or other detention facilities if they are caught using mobile phones to contact their family members who have fled abroad.

The North has begun its intensive surveillance at the border town of Hoeryong and has been expanding its crackdown into other areas, such as Musan and Onsong, the sources added.

Ahead of the party event, the North is also pressing North Koreans hard in its "70-day campaign of loyalty," ordering them to present rice to the regime, according to another source who has knowledge about North Korea.

Since Friday, North Korea has ordered every house to donate some 1 kilogram of rice to the country, drawing complaints from North Koreans, the source added. (Yonhap)