Vladimir Putin has accused Ukraine of practicing "terror” over an alleged border clashes that he said left two Russian servicemen dead, in the worst escalation of tensions between the two countries in months.

Russian security agencies said on Wednesday that two Russians were killed as they thwarted Ukrainian commando raids into Crimea over the weekend.

Ukrainian officials angrily denied any such incident had taken place and accused Mr Putin of seeking a pretext for war, in an exchange that raised fears of a renewal of the levels of confrontation that marked the annexation of Crimea and outbreak of war in east Ukraine in 2014.

"From the Russian side there were losses - two soldiers killed. We obviously will not let such things slide by," Mr Putin told reporters in Moscow on Wednesday afternoon.

"The attempt to provoke an uptick in violence, to provoke conflict is nothing but an attempt to distract public attention," he said.

He added that he saw no point in holding planned talks with Petro Poroshenko, the president of Ukraine, at the G20 summit in China next month.

Russia’s Federal Security Service, which controls the country’s border guards, said one of its officers and one solider were killed in separate cross border incursions by Ukrainian forces on Sunday and Monday.