Russian President Vladimir Putin has warned NATO over its possible transfer of F-16 fighter jets to Ukraine, as Ukrainian forces intensify their counteroffensive.

He gave the warning during his speech at an international conference in the Russian city of St. Petersburg on Friday.

Putin said Russian forces had destroyed a number of German-made Leopard tanks. He added that they would also destroy the F-16 jets if they were supplied to Ukraine.

Ukraine has repeatedly pleaded for Western fighter jets.

Putin said if the jets were deployed at bases outside Ukraine to be used in hostilities, his country would have to consider where and how to destroy those weapons.

He warned "There is a serious danger of NATO being further drawn into this armed conflict."

Putin is scheduled to meet with the leaders of seven African countries, including South Africa and Egypt, in St. Petersburg on Saturday. They are expected to exchange views on the situation in Ukraine.

The delegates visited Kyiv, the Ukrainian capital, the previous day to mediate between Ukraine and Russia.

Meanwhile, Ukrainian forces are continuing their counteroffensive in the eastern and southern regions of Donetsk and Zaporizhzhia.

Ukraine's deputy defense minister, Hanna Maliar, said on social media on Friday that Ukrainian units had achieved tactical successes and gradually moved forward in the country's south.

But she said the Ukrainians were facing fierce resistance as Russian forces increased their dominance in the air.

The British defense ministry noted in its update on Saturday that Russia had reinforced its attack helicopter units in southern Ukraine.

The ministry said imagery showed that more than 20 extra helicopters had been deployed to an airport facing the Sea of Azov, about 100 kilometers behind the front line.

The ministry added that "Russia has gained a temporary advantage" in the region.