Russia's Foreign Ministry on Tuesday criticised a decision by US authorities not to press charges against the adoptive parents of a Russian-born boy who died in January, saying it "raised serious questions".

The death of three-year-old Max Shatto led to criminal and child welfare investigations in the US state of Texas, where he died. Russia has also opened an inquiry and lawmakers in Moscow called for his younger brother to be returned to Russia.

The dispute over the death has also caused new strains in US-Russian ties.