UPDATE 1-North Korea delays access to Kaesong industrial zone

* N.Korea delaying access to joint industrial complex

* 861 South Korean workers currently in Kaesong complex

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SEOUL, April 3 (Reuters) - North Korea has delayed the daily opening of its Kaesong industrial zone with South Korea in a move that could represent a sharp escalation of tensions between the two countries and potentially trap hundreds of South Koreans in the North.

The daily entry clearance, agreed by telephone, to the joint complex had been delayed for over an hour early on Wednesday. The North had previously threatened to close the complex as part of a standoff with Washington and Seoul.

"We are waiting for access from the North Korean authorities," a Unification Ministry official said. The ministry said 861 South Korean workers are in the industrial complex while 179 workers await entry.

The complex is a rare lucrative source of income for the impoverished North since it was established as a form of joint-Korean cooperation in the early 2000s.

The latest move comes after North Korea said on Tuesday it would revive a mothballed nuclear reactor able to produce bomb-grade plutonium in a standoff that has seen Washington shift military resources into South Korea.

Pyongyang has been ramping up its threats since it was hit by international sanctions following its third nuclear test earlier this year.

(Reporting by Christine Kim and Ju-min Park; Editing by David Chance and Dean Yates)