Sex ratios at birth are becoming increasingly imbalanced in Vietnam, with far more boys being born than girls, the UN Population Fund says.
For every 100 females, 110.6 males were born - compared to a norm of 105.
The situation was particularly worrying because of the rapid increase in the proportion of boys being born in the last five years, it said.
The UNFPA warned that the imbalance could lead to a number of social problems in the coming years.
In May 2009, Deputy Prime Minister Nguyen Thien Nhan warned that the gender imbalance in Vietnam could lead to about 3 million men having difficulty in finding wives by 2030.
Bruce Campbell, the UNFPA's representative in Vietnam, said that other Asian countries with gender imbalances had developed these over much longer periods.
"Over 30 years China reached the ratio of 130 (males per 100 females) and Korea 116, and these are declining," he said.
"Vietnam went from quite a normal level of 105 to 110.6 in the last 5 years."
Vietnam banned foetal sex selection in 2003 in an effort to tackle this problem, but the practice is still going on.
"Three factors that contribute to the increase of an imbalanced ratio at birth are firstly son-preference, a very fundamental aspect of culture and society in many countries, secondly the pressure of fertility to have a smaller family size, especially in a number of Asian countries, and thirdly the access to legal and affordable technology for son-selection," Mr Campbell explained.
"And in Vietnam it's the combination of all three factors."
Dr Tran Van Chien, vice-head of Vietnam General Office for Population and Family Planning (Ministry of Health), says that the biggest difficulty Vietnam was facing in tackling the problem was the 1,000-year-old tradition that favours men over women, where men carry on the family line and care for elderly parents.
He says changing this will take years.
But he said Vietnam was determined to learn from other countries' experience in raising awareness among its population, coupled with policies to enhance women's status in society.