Washington is trying to help hundreds of American families whose dreams of adopting a child from Russia are in limbo after the passage of a controversial Russian law, a top US official said Tuesday.

Calling it "tragic" for the families and children involved, State Department spokeswoman Victoria Nuland said some 500 to 1,000 US families already had made progress in their adoption plans before the new ban came into force.

In many cases the families have met with the children they planned to offer a new home in the United States, and already bonded with them.

"We are continuing our conversations with the Russian government. We would obviously like to see those adoptions be able to move forward," Nuland said.

"But, you know, we have now been informed by the Russian government that they are going to formally suspend our agreement, but we're going to continue to try to work on these pipeline cases."

Moscow's ban on adoptions of Russian children by American families came into force on January 1, just days after it was signed into law by President Vladimir Putin sparking an international outcry.

The ban is part of a law rushed through parliament to hit back at the United States over its passing of a law sanctioning Russian officials implicated in the death in jail of lawyer Sergei Magnitsky in 2009.

But opponents say it makes Russian orphans -- many with physical or mental difficulties -- the blameless victims of a diplomatic standoff between Washington and Moscow.

US families have adopted more than 60,000 Russian children over the past 20 years, but the annual figures have dropped to just under 1,000 in 2011 from a high of over 5,800 in 2004.

Families who were already in the process of adopting a Russian child should contact the US government via a special website www.adoption.state.gov "so that we can have a full picture," Nuland told journalists.

Earlier, the US State Department described the ban on adoption of Russian children by Americans as "politically motivated" and said Washington deeply regretted the move.