SEOUL—For decades after North Korea’s founding in the 1950s, financial security wasn’t a major concern for its citizens. A communist system provided most daily needs, and for many years living standards outstripped those of South Korea.

Then a devastating famine in the 1990s and a subsequent economic collapse crippled the public distribution system and forced citizens to fend for themselves.

The semimarket economy that emerged has expanded rapidly in recent years, providing a living for up to three-fourths of the country, according to observers, defectors and those with contacts in a state that is largely closed off from the rest of the world. As unauthorized private commerce has bolstered North Koreans’ incomes, an unregulated system of lending and currency exchange has also emerged, they said.

“People are investing and are making money,” says Kim Young-hui, a former North Korean banking official and an analyst on the North Korean economy at KDB Bank in Seoul. Observers such as Ms. Kim, who left North Korea in 2003, say refugees from all provinces have reported private lending activities in their respective hometowns.

There is no reliable study to measure precisely how much money is flowing in and out of the country, observers and defectors say, and North Koreans remain excluded from anything close to a modern financial system. There are no commercial banks and little trust in the state for economic security.

Defectors’ accounts depict a system in which private savings are being funneled into lending to generate a profit, but without any legal framework or guarantee on investments. Scams occurred in the early years but in the last three years, a culture of credit has settled in, says Lim Eul-chul, a professor at Kyungnam University’s Institute for Far Eastern Studies in Seoul who studies the North Korean economy.

“As businesses expand, the importance of credit goes up. Now you are doing business with more than just people you know,” said Mr. Lim, adding that friends or family often guarantee payment.

According to observers and defector accounts, some North Korean lenders and investors are funding seed and fertilizer purchases in return for a cut from the following year’s harvest, or lending money to merchants to import goods that range from apples to Italian luxury goods. They are borrowing and lending with interest, dodging North Korea’s ban on usury. Even state traders are borrowing money from them, some defectors say.

Pawn shops have opened up under official blessing and people leave anything of value for small loans, they say. Some private-property ownership, such as apartments, has been allowed since 1998, and some lenders independently appraise the value of real-estate properties to collateralize larger loans. A default on a loan can lead to confiscation of goods by the private lenders, they say.

It is impossible to independently verify accounts of private commerce and lending inside the reclusive country. But since taking power at the end of 2011, North Korean dictator Kim Jong Un has promised to improve living standards and appears to have taken a relatively laissez-faire approach to market activity.

Such activity—from selling toothpaste to buying 3G-enabled smartphones on the street—still isn’t officially allowed in North Korea, but the regime often turns a blind eye to the commerce, some observers say. The regime even appears to be collecting some money from the activity, in the forms of bribes to authorities or fees for securing a market stall, they said.

North Korea observers such as Andrei Lankov, a professor at Kookmin University in Seoul, say that between half and three-quarters of North Korea’s household income comes from the private sector. Those who have some money aren’t just lending, they are opening up restaurants, computer stores, karaoke shops or communal bathhouses. Private taxi and delivery services have emerged in recent years.

Those who engage in commerce are reaching consumers and intermediaries more than before, thanks to the recent proliferation of cellphones. Buyers and sellers no longer have to exchange cash in person, they can tell their financiers to settle the bill.

The regime has kept track of these developments and has tried to regulate private wealth, albeit with little success, some observers say.

Trust in the state took a heavy blow following a 2009 currency devaluation that caused a panic over the sudden loss of wealth. The policy damaged the flourishing black market and reinforced the public’s preference for foreign currency. Now, most private deals are in U.S. dollars and Chinese renminbi, observers say.

“In large cities like Chongjin [a port near the northeastern tip of North Korea where China and Russia meet], it’s 100% renminbi,” says one former North Korean farmer, who fled to South Korea in 2013.

A recent attempt by the regime to introduce a degree of personal finance came with the introduction of a card-payment system. Kim Chon Gyun, president of the central bank, told Japan-based newspaper Choson Sinbo in February that North Korea is working to fuel economic development by “smoothly circulating internal capital.”

Mr. Kim said the country was developing an unspecified “new financial product” and was encouraging the use of bank cards in ordinary people’s lives, in a rare public disclosure of economic policy.

Peoples’ accounts vary, but they say North Korea since 2011 has been issuing at least two kinds of chip-bearing plastic cards that allow users to load domestic or foreign currencies. The cards can be used for payment at some stores, and the salaries of some government officials appear to be wired to such accounts, they say. They are being used in provinces as well as in the capital.

But some observers say the system’s adoption among ordinary citizens appears low, and its success is contingent upon conditions such as a guarantee of deposit and the regime’s assurance that the origin of the money won’t face scrutiny.

Write to Jeyup S. Kwaak at jeyup.kwaak@wsj.com