North Korea could "additionally supply troops, weapons and ammunition going forward", said the official, who spoke on condition of anonymity.
Seoul, Kyiv and Washington have all said that North Korea sent more than 10,000 troops to Russia last year to help the Kremlin in its war against Ukraine.
Ukraine previously said it had captured or killed several North Korean soldiers in Kursk, France 24 reported.
President Volodymyr Zelensky has also published footage of interrogations with what he said were North Korean prisoners captured by the Ukrainian army on the Kursk front.
Neither Moscow nor Pyongyang have confirmed the deployment.
However, the two countries signed an agreement, including a mutual defence clause, when Russian President Vladimir Putin made a rare visit to the nuclear-armed North last year.
Ukraine's military also said it believed North Korean soldiers deployed to the front line in Kursk had been "withdrawn" after suffering heavy losses.
A report by Seoul's defense ministry, submitted to the South Korean parliament's defense committee, warned on Tuesday that the North was "continuing to provide weapons, ammunition and other military support to Russia following its troop deployment in the Ukraine war".
Seoul is "closely monitoring" whether Moscow could "transfer advanced military technologies to North Korea in return for this support", it said.
MA/PR