Offenders can be punished with hours of forced labor or fines of 700 won, almost a week's salary for the average worker, human rights group Good Friends said, citing its own sources within the isolated nation.
The Stalinist leadership's campaign is angering women who see skirts as less practical than trousers, Good Friends director Lee Seung-Yong said.
"Women are told to wear skirts in public places and in the streets, sparking complaints among them as they often have to work in tough conditions," he told AFP.
Disciplinary officials from students' bodies and women's organizations stand at street corners during the morning rush hour and lunch breaks, to keep watch for any women violating the pants ban, according to Good Friends.
Uriminzokkiri, an official North Korean website, noted on Monday that ruler Kim Jong-Il had issued a decree in 1986 urging women to wear traditional Korean attire.
"The Dear Leader has said national character shows up not only in language, etiquette and morals but in attire as well," the site said.
It quoted Kim as saying the country's traditional skirts and jackets are a "source of our (national) pride" and that women should be "actively encouraged" to wear them.