He added that its delay allowed Russia to strengthen its defenses.
"We had plans to start in the spring. But we didn't, because, frankly, we didn't have enough ammunition and weapons and we don't have enough brigades that are adequately trained for these weapons, in addition, the training missions were held outside Ukraine," Zelensky said, the Brussels portal Politico reported.
South Africa's president, in Russia as part of a delegation pushing for peace between Kiev and Moscow, told his Russian counterpart Vladimir Putin on Saturday that the fighting had to stop.
His delegation put forward a set of principles that the Kremlin deemed "very difficult to implement", a day after Ukrainian President Volodymyr Zelensky ruled out talks with Moscow.
The long-announced Ukrainian counter-offensive has been underway for several days, and the US, Great Britain, Denmark and the Netherlands are sending hundreds of missiles to Ukraine. At the same time, NATO defense ministers are discussing in Brussels the continuation of the support that the allies have been providing to Ukraine for months.