Border crossings by South Koreans working at the Gaeseong industrial complex in North Korea proceeded without incident on Thursday amid growing concern over their safety following Pyongyang’s severance of a military hotline on the previous day.

In the morning, three groups of South Koreans crossed the land border separately through the Customs, Immigration and Quarantine office in Paju, north of Seoul, Seoul officials said.

They received entry approval by phone from a North Korean complex management office in Gaeseong. A total of 530 South Koreans were to travel to the complex while 511 people were to return home.

Criticizing Seoul and Washington for being “hostile,” Pyongyang on Wednesday abruptly cut the military hotline to ensure the safety of South Korean workers commuting to and from the complex.

On average, 700 to 800 South Koreans stay at the industrial complex that offers a crucial source of hard currency for the isolated, impoverished state.

Some 120 South Korean companies are in operation there, capitalizing on the cheap labor.

The presidential office of Cheong Wa Dae stressed it prioritized the safety of South Koreans working in the complex, noting that some 1,300 regular phone and fax lines linked to the industrial complex were still operating as normal.

“For now, there are not any unusual movements at the complex. With the Unification Ministry at the center to ensure their safety, we are closely watching the situation there,” a senior Cheong Wa Dae official told reporters.

“President (Park Geun-hye) has ordered that the safety of South Koreans there be prioritized before anything else.”

Inter-Korean exchanges have plummeted since May 2010 following the sinking of the corvette Cheonan by North Korea. The Gaeseong complex, however, has continued operations.

Some 53,000 North Koreans are working at the complex with a monthly average pay of $144. Through the complex, the cash-strapped country rakes in more than $90 million each year.

By Song Sang-ho (sshluck@heraldcorp.com)