( CNN ) -- North Korea fully reopened its border to South Koreans on Tuesday , without explaining its reversal , the South 's Unification Ministry said . Cross-border gates between North and South Korea were closed on March 9 . The communist nation shut its border on March 9 , calling 12-day U.S.-South Korean military exercises a threat to its safety . But on Tuesday , North Korea normalized visits by South Korean workers and cargo trucks to an industrial complex jointly run with the South , Yonhap said . `` The North Korean side sent us a letter of approval this morning , '' the news agency quoted Unification Ministry official Lee Jong-joo as saying . The letter did not explain the North 's reversal , Lee told Yonhap . Two batches of workers , totaling 287 people , crossed the border into North Korea on Tuesday , according to the Unification Ministry . More than 300 were scheduled to return at the end of the workday . North Korea had partly opened its border Monday , allowing nearly 300 South Koreans to return home . Many of the South Koreans work at the industrial complex in the North Korean border town of Kaesong . The border closing was part of a recent series of aggressive moves by North Korea , which included cutting the last remaining communications channel between North and South ; saying that the North could not guarantee the safety of South Korean passenger jets in its airspace during the annual joint military exercises ; and threatening to retaliate if a `` satellite '' launch from its northeastern coast was intercepted , saying interference would `` mean a war . '' U.S. and South Korean officials have said that North Korea appears to be preparing to test-fire its long-range missile , the Taepodong-2 , under the guise of launching a satellite into space . The missile is thought to have an intended range of about 4,200 miles ( 6,700 kilometers ) that , if true , could give it the capability of striking Alaska or Hawaii .
NEW : North Korea fully reopens its border to South Koreans without explanation . U.S.-S. Korea closed border March 9 because of North Korea military exercises . Border partly opened March 9 , allowing 300 South Koreans allowed to go home . Stranded South Koreans mostly work at joint industrial complex in border town Lee Jong-joo .