UNITED NATIONS - With the world population reaching a milestone of seven billion on Monday, UN Secretary- General Ban Ki-moon called for international solidarity as the globe faces a multitude of crises.

"Today the world's population reached seven billion," Ban said at a press conference at the UN Headquarters in New York to mark the milestone. "The world's population reached six billion in 1998, 13 years ago. It is expected to grow to 9 billion by the middle of this century, or even a few years earlier - by 2043."  

"This is a day about our entire human family," said Ban.

"Our world is one of terrible contradictions," Ban said. "Plenty of food but one billion people go hungry. Lavish lifestyles for a few, but poverty for too many others. Huge advances in medicine while mothers die everyday in childbirth, and children die every day from drinking dirty water. Billions spent on weapons to kill people instead of keeping them safe."

Also on the special occasion, he drew the world attention to global scourges such as the economic and food crises.

Ahead of the G20 summit in Cannes, France, Ban said his message will be "loud and clear - think about our children... Think about the future, with vision and foresight".

"Yes, we face a serious economic crisis," Ban said. "For much of the world, fiscal austerity is the new order of the day. Yet even in these difficult times, we cannot afford to cut loose those who are hardest hit."

Ban said the G20 summit must deal with all these issues "squarely and directly".

"Today, we welcome baby seven billion," Ban said. "In doing so we must recognize our moral and pragmatic obligation to do the right thing for him, or for her."

Last week, the United Nations Development Fund (UNFPA) released the report titled "The State of World Population 2011", ahead of the milestone of seven billion, which said as the population grows, so do challenges facing humanity.

According to the report, the population record is considered a success, because it means that people throughout the world can live longer. However, with the current growth, the world will have more 78 million people each year, leading to increasing demands on natural resources and big pressure on the planet.

Currently, 1.8 billion of the world's seven billion are young people between the ages of 10 and 24, 60 percent are under 25 years old, said the UNFPA report.