SHANGHAI, July 23 (Xinhua) -- Authorities in China's largest city, Shanghai, have launched a campaign to encourage couples who are themselves only children to have a second child, as concerns mount about a rapidly aging population.

    Officials would visit these families, publicize the policy allowing a second child and provide consulting services, said Xie Lingli, director of the Shanghai Population and Family Planning Commission, on Thursday.

    The campaign was aimed at reducing the ratio of the elderly and preventing future labor shortages, Xie said.

    Shanghai families each with only one child account for 97 percent of the total in the city of nearly 19 million, according to Xie's commission.

    "Shanghai has about 3 million people aged 60 or older, 21.6 percent of the population. The rising number of retirees will put pressure on the younger generation and the social security system," Xie said.

    Divorced Shanghai residents are allowed a child with a new spouse even if they already have one from a previous marriage. Disabled parents whose ability to work is impaired can also have more than one child.

    Since the city cancelled a four-year interval between the first and second births in 2004, more couples are having a second child. Births of second children rose from 2,910 in 2005 to 3,934 in 2007, according to the commission.

    The campaign never means the change of the national and local family planning policies and regulations, said Xie, stressing that only eligible couples are encouraged to have a second child.

    China's one-child policy was enacted in 1979 and was incorporated in the Population and Family Planning Law in 2002. Officials say it will be strictly enforced as a means of controlling births for decades to come as overpopulation is still a major concern.

    Shanghai's population is expected to surpass 19.5 million in 2010 and 23 million in 2020, Xie said.