The head of the British secret intelligence agency known as MI6 says he believes Russia's invasion of Ukraine is about to lose steam.

Richard Moore was speaking at a security forum organized by a US think tank in Colorado on Thursday.

Moore said the Russian military would find it increasingly difficult to supply troops and material over the next few weeks. He added that the supply challenges are likely preventing Russian forces from making significant advances in the face of Ukrainian counterattacks.

He said they will have to pause in some way, giving the Ukrainians opportunities to strike back.

He said effective Ukrainian counterattacks will be "an important reminder to the rest of Europe that this is a winnable campaign by the Ukrainians."