Ukrainian troops have been on the offensive in the country's south in a bid to retake areas seized by Russia.

Ukrainian forces are mainly targeting Russian ammunition depots and resupply routes by making use of rocket systems and other weapons provided by the United States and European countries.

Britain's Defence Ministry said on Saturday that the two primary road bridges giving access to Russian-occupied territory on the west bank of the Dnipro River in the southern region of Kherson "are probably out of use" as a result of Ukrainian strikes.

The ministry said it is almost certain that Russia now has to rely on just two pontoon ferries to resupply several thousand Russian troops on the west bank, which is constraining the supply chain.

A senior US government official on Friday reiterated that preparations are underway for Russia to hold referendums on annexation in Kherson, the southeastern region of Zaporizhzhia and the eastern regions of Luhansk and Donetsk. The official said there are moves for a referendum in the eastern region of Kharkiv as well.

The US official also said the voting could take place in the coming weeks, and vowed that Washington will respond swiftly and sternly by imposing additional sanctions against Moscow if it proceeds with the plans.

Pro-Russian separatists in those regions say the referendums should be held on September 11, to coincide with the local elections in Russia.

Meanwhile, the Institute for the Study of War, a US think tank, said in its latest assessment that Ukrainian partisans are likely targeting Russian occupation officials and Ukrainian collaborators who are preparing for the referendums to disrupt the Russian annexation of occupied territories.