Russian president Vladimir Putin signed a bill on Friday banning American families from adopting Russian orphans, apparently in retaliation for U.S. criticism of his nation's human rights record, Fox News reports. The law will block dozens of Russian children expected to be adopted by American families from leaving the country and cut off one of the main international routes for Russian children to leave orphanages. Russia is the single biggest source of adopted children in the U.S., with more than 60,000 Russian children being taken in by Americans over the past two decades. According to UNICEF estimates, there are more than 700,000 Russian orphans but only 18,000 Russians waiting to adopt a child. The U.S. State Department previously expressed deep concern about the Russian measure. "The welfare of children is simply too important to be linked to political aspects of our relationship," said spokesman Patrick Ventrell.