Edwin Salvador, director of the WHO office in North Korea, said among the 1,211 people tested, 696 people were quarantined, Radio Free Asia's Korean service reported.
Pyongyang has claimed since January there have been zero cases of the novel coronavirus, but said this week a defector who had recently re-entered the North was a suspected COVID-19 patient.
Seoul has said the defector, who may have swum across the border, was not on its list of confirmed patients, and analysts in the South say North Korea could be confronting a COVID-19 outbreak.
The nearly 700 North Koreans under quarantine are workers who were based in border regions, including at Nampo Port and in the city of Sinuiju, which faces the Chinese city of Dandong, in Liaoning Province, Salvador said.
The report of quarantined workers in North Korea comes after the WHO had said on July 9 Pyongyang had isolated 610 people after conducting 1,117 COVID-19 tests. The RFA report indicates North Korea increased testing in July after claiming the state had brought the disease under control.
According to Salvador, North Korea may have stepped up disease control in the wake of new cases in China. The two countries share an 880-mile border.
North Korea is also placing greater restrictions on foreigners in Pyongyang, including diplomats in the capital.
The Russian Embassy in North Korea said on its Facebook page on Wednesday that Pyongyang's foreign ministry sent a notice to diplomats, urging them to report any "suspicious symptoms," including fever and coughing, to health authorities at Pyongyang Friendship Hospital. North Korea also urged diplomats to cease imports of products from their home countries.
Diplomats were also advised against traveling outside the capital, according to the embassy.