"As the world's major military power, the United States can really make a difference on this issue," said Jo Becker, children's rights advocate for Human Rights Watch. "As a party to the treaty, the United States will be better able to use its considerable political and military influence to discourage the use of children as soldiers by other governments and armed groups."

The treaty, formally known as the Optional Protocol to the Convention on the Rights of the Child on the involvement of children in armed conflict, sets 18 as the minimum age for forced recruitment and for direct participation in hostilities. It allows countries to accept voluntary recruits as young as 16 with certain safeguards, including parental permission.

The United States initially opposed an 18-year minimum age for combat. In the past, the United States has sent 17-year old troops into armed conflicts in Somalia, Bosnia and the Gulf War. The Pentagon has now agreed to change deployment practices to comply with the protocol, by taking "all feasible measures" to ensure that 17-year old soldiers do not directly participate in hostilities. Less than one-quarter of one percent of U.S. soldiers are still under the age of 18 when they complete their training.

Earlier this week, the U.N. secretary-general issued a report on children and armed conflict, which singled out parties in five countries currently under consideration by the Security Council for the recruitment and use of child soldiers: Liberia, the Democratic Republic of Congo, Burundi, Somalia and Afghanistan. Other countries with widespread use of child soldiers include Burma, Colombia and Northern Uganda. Globally, an estimated 300,000 children under the age of 18 are currently fighting in more than 20 conflicts worldwide.

"The use of children in combat offends the most basic standards of decency," said Becker. "U.S. ratification of the protocol sends a clear message that this practice cannot be tolerated."