The Chinese government has begun requiring sterilization of couples with two or more children because efforts to limit growth of the world's largest population have stalled.

The requirement, coming after the second straight year of stubbornly high birth rates, is believed by experts to be the harshest population control measure in the world. U.N. officials who coordinate a $50 million family planning project for China say they are investigating whether the new policy violates the U.N. ban on funds for programs forcing sterilization.

"If there is a very explicit regulation that all couples with a second child must be sterilized," said a U.N. official here, "it could cause serious problems for the United Nations."

The new guidelines reportedly began in January but were not publicly aired until May 14 in a radio broadcast from south China. The broadcast called for "compulsory sterilization" for one of the parents of a two-child family.

"The stipulation providing that either party, husband or wife, of those couples with two children must undergo a sterilization operation is an important technical policy laid down by the state Family Planning Commission," said Guangdong Province Deputy Gov. Wang Pingshan, who was quoted in the broadcast.

Although official pressure and fines have been wielded to encourage abortions and insertion of contraceptive loops, Chinese leaders previously stressed the voluntary nature of sterilizations.

Family Planning Commission spokesman Wang Liancheng confirmed the new intensified drive for sterilizations, but he insisted the policy is not intended as a license for imposing the surgical procedures through coercion.

Wang said the state merely "recommends" sterilizations as the most effective birth control measure for couples who already have two children.

Nevertheless, he said, urban dwellers who ignore the recommendation could be fined up to 10 percent of their wages. Dissenters in rural areas stand to lose part of their land or suffer monetary penalties, depending on their birth-control obligations outlined in agreements with village leaders.

"We are getting stricter," he said in an interview today.

Wang said the state decided in January to "stress" sterilizations after discovering unacceptably high birthrates among couples with at least two children. In 1981, for example, almost 30 percent of total births reportedly filled that category.

The problem is most severe in China's vast countryside where, according to Wang, "experience shows that the simple methods are best. For peasants with two children, the best way is sterilization of one of the parents.

"That is required by the state, but that doesn't mean we have excluded other methods," said the spokesman.

In every case, however, force should be ruled out, he said. "No operation can be done by binding up a person and carrying him or her to a hospital for sterilization," he said. "No operation can be done without the person's consent."

Limiting growth of China's 1 billion people has become a cornerstone of the Communist government, which considers birth control a prerequisite for the nation's modernization.

With an ambitious goal of containing the population at 1.2 billion by the year 2000, officials have fashioned a strict policy of fines and rewards designed to discourage couples from having more than one child.

But tradition-bound Chinese who value children as an insurance policy for the later years have handed the government setbacks. Far from the target of .95 percent annual growth rates, China's population has leaped ahead at the rate of 1.45 percent since 1980.

Couples with at least two children have been among the principal offenders, contributing 5.8 million births in 1981.

Despite reports of forced abortions and heavy-handed tactics by local birth-control cadres, Chinese officials consistently deny their policies are coercive.

"Without voluntary cooperation from the people, it would be unthinkable that birth control can be practiced in this country of 1 billion," Family Planning Minister Qian Xinzhong said after winning the first U.N. population award in March.

China's media widely publicized Qian's award and has relied on the United Nation's $50 million contribution to help finance its census last year, demographic research, contraceptive production and training of family planning personnel.

U.N. officials based here said they were alarmed by the broadcast reports--monitored in Hong Kong--of compulsory sterilization because of the U.N. proscription on compulsion in these matters.

"Compulsion in these matters is not acceptable," said one official.

In India, revulsion against administrative excesses that led to widespread compulsory sterilization contributed to the downfall of Prime Minister Indira Gandhi in 1977.