A Kremlin official has said that a bilateral agreement with Washington on adoptions is still in place, creating uncertainty over the immediate impact of Russia's new ban on US citizens adopting Russian children.

Russian President Vladimir Putin approved the ban from Jan 1 as part of a law passed to retaliate against legislation in the United States intended to punish Russian human rights abusers.

Moscow had told Washington it was also terminating a bilateral agreement regulating adoptions — but Putin's spokesman on Thursday said a technicality meant the agreement would have to remain in force until the beginning of 2014.

"The agreement is still in effect," Dmitry Peskov told the RIA news agency, citing a built-in, one-year delay to any terminations.

He confirmed his statement to Reuters, and said it would be up to legal experts to determine what effect it might have on US families who were still going through the process of adopting Russian children when the ban was imposed.

The US State Department said it was unclear about the impact of Peskov's statement, but told reporters it was "very hopeful" it would be able to work through adoption cases that had already begun.

US officials were sifting through e-mails from about 950 US families to establish where they were in the adoption process, said State Department spokeswoman Victoria Nuland.

A prominent Russian lawyer, Genri Reznik, said that because an article of the Russian constitution says international treaties take precedence over Russian laws if they contradict one another, Peskov's announcement meant adoption procedures could continue for a year.

But Peskov added to the uncertainty in later interviews by suggesting that only children whose adoptions have been approved by Russian courts would be able to go to the US.

"In cases where certain legal procedures have not been completed, a full ban on adoptions by parents from America takes effect," Peskov told the Internet and cable TV channel Dozhd.

"The (Russia-US) agreement is in no way a mechanism that obliges the Russian side to give its children up for adoption," he added.

Peskov gave no numbers on Thursday, but he was quoted as saying in late December that six adoptions that had been approved would go through, while another 46 that were under way would not. He was not immediately available to comment further on Thursday.

Russian lawmakers have said the adoption ban was justified by the deaths of 19 Russian-born children adopted by US parents in the past decade.

But children's rights activists have accused the Russian government of making vulnerable children pawns in a political dispute. Opponents of Putin are planning a protest march over the law in Moscow on Sunday.