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March 16 (Reuters) - North Korea's repeated blocks on traffic crossing the border into an industrial estate run by South Koreans has thrown into doubt the future of one of the communist state's few guaranteed foreign currency earners.

Here are some key facts about the Kaesong industrial park:

LOGISTICS

The park is near the North Korean city of Kaesong about 70 km (45 miles) northwest of Seoul. It is the first and only such venture between the two countries, which remain technically at war.

South Korean firms, with some financial incentives from their own government, have set up factories in the park mostly to assemble products using cheap North Korean labour. Despite attempts to lure them in, no other foreign companies have yet set up operations there.

Materials and the finished products move along a new highway and a restored rail link that crosses the 4-km Demilitarised Zone that has acted as a buffer between North and South since the end of the 1950-53 Korean War.

It is run by Hyundai Asan, part of the Hyundai Group, along with Korea Land Corp.

EMPLOYMENT

About 38,000 North Koreans are employed at 101 South Korean factories producing or assembling items such as textiles, watches and cosmetic cases.

The minimum monthly wage of $70 is paid to the North Korean state and not directly to workers.

Most of the workers are bussed in from Kaesong City but there had been discussions about bringing workers from elsewhere in the North to meet future demand when the park is expanded.

CHARGES OF EXPLOITATION

The United States and other governments have in the past raised concerns about possible worker exploitation at the complex and whether it may simply be another means to channel funds to the North Korean government.

DUTY-FREE EXPORT?

South Korea and the United States agreed to set up a joint committee to study allowing Kaesong products duty-free status in the U.S. market under a free trade deal struck in April 2007. South Korea says future projects in the North will be entitled to the same privilege. Washington is less enthusiastic.

South Korea wants goods produced in Kaesong to be covered in other free trade deals.

FUTURE PLANS

South Korea's vision for the Kaesong project, which began in June 2003 and saw the first batch of goods shipped to South Korea in 2004, includes more than half a million North Koreans employed by 2,000 firms and with hotels, golf courses and a "peace park". (Sources: Reuters, South Korean Unification Ministry, Hyundai Asan) (Reporting by Seoul bureau)

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