WASHINGTON (AFP) - US Secretary of State Mike Pompeo said on Tuesday (Aug 25) the United States is "deeply concerned" about the apparent poisoning of Kremlin critic Alexei Navalny and urged a full investigation.

"The United States is deeply concerned by reported preliminary conclusions from German medical experts that Russian opposition activist Aleksei Navalny was poisoned," Mr Pompeo, who is in the Middle East, said in his first comments on Mr Navalny.

"If the reports prove accurate, the United States supports the EU's call for a comprehensive investigation and stands ready to assist in that effort."

Mr Navalny fell ill last Thursday on a flight in Siberia, and was treated in the region for two days before being airlifted to Berlin.

Doctors there said on Monday that clinical tests "indicate poisoning with a substance from the group of cholinesterase inhibitors", a kind of nerve agent.

"Mr Navalny's family and the Russian people deserve to see a full and transparent investigation carried out, and for those involved to be held accountable," Mr Pompeo said.

Earlier the US ambassador in Moscow, John Sullivan, said the Russian authorities should open an "immediate, comprehensive, and transparent investigation" into the case.

But the Kremlin has dismissed the findings of the German doctors.