* Kaesong employs more than 50,000 N.Korean labourers
* S.Korea yet to confirm report that North Koreans didn't show up for work
* Tension builds into one of worst crises since end of Korean War in 1953 (Adds details and background throughout, factory manager paragraphs 11-12)
By Christine Kim
SEOUL, April 9 (Reuters) - North Korean labourers did not report to work early on Tuesday at the Kaesong industrial zone it jointly operates with South Korea, Yonhap news agency reported, a day after Pyongyang said it would withdraw workers and suspend operations indefinitely.
Isolated and impoverished North Korea suspended work at Kaesong, its sole remaining major project with the south, on Monday amid what has become one of the most serious crises on the Korean peninsula since the end of the Korean War in 1953. [ID:nL3N0CV1P}
It is the first stoppage since the Kaesong industrial project began shipments in 2004.
Tensions have been rising since the United Nations imposed new sanctions against the North after Pyongyang carried out its third nuclear test in February.
Speculation has increased it will carry out some sort of provocative action -- either another nuclear test or a missile launch -- deeply worrying South Korea and its major ally the United States.
A South Korean government official could not immediately confirm the Yonhap report on Tuesday and said authorities were investigating.
The Kaesong complex employs more than 50,000 North Koreans and is one of the North's few sources of ready cash.
On Monday, the North's official news agency KCNA cited senior official Kim Yang Gon as saying North Korea would decide later whether it would continue to operate the zone.
About 475 South Korean workers remain in Kaesong a week after North Korea banned all South Koreans from entering the complex. The South's Unification Ministry said 77 South Korean workers were expected to return home on Tuesday.
Thirteen factories have stopped operations in Kaesong due to lack of raw materials, according to the Unification Ministry. A total of 123 South Korean companies generate more than $80 million a year in cash in wages at the complex.
An executive at a South Korean apparel firm running a factory in Kaesong said late on Monday his employees had told him they would stay on at the factory.
"I don't know what to do, honestly. I can't simply tell my workers to leave or stay," said the executive, who requested anonymity. (Additional reporting by Ju-min Park; Editing by Paul Tait)