Five weeks into Kyiv's long-anticipated operation, much of the front appeared to be frozen.

Presidential advisor Mykhailo Podolyak admitted progress was "slower than we want".

"Undoubtedly, this operation will be quite difficult, long and will take quite a lot of time," Podolyak said.

Podolyak also said Kyiv was seeking a joint military patrol of Black Sea countries in order to continue grain exports from its ports after Russia exited a landmark deal ensuring the safety of cargo ships.

He spoke two days after Moscow left the deal and as Russia pounded Ukrainian Black Sea port infrastructure for two straight nights.

"Negotiations are ongoing at all levels. A UN mandate should be added here to create a military patrol that would include countries in contact with the region, for example Turkey, Bulgaria or any others," Podolyak said.

On the battlefield, Podolyak said Kyiv's offensive was difficult because of heavily mined territory as well as logistical problems, including in arms deliveries.

He said Kyiv needs an additional 200 tanks and dozens of F-16 fighter jets to accelerate its efforts to wrest Russian forces from entrenched positions in the south and east of the country.

"Of course, we need additional armoured vehicles -- two to three hundred tanks first of all," Podolyak told AFP journalists.

"We need 60 to 80 F-16 planes in order to close the skies well, especially in the area near the front."

But he claimed that the number of "offensive operations" led by the Ukrainian army was "progressively rising" and said that not all brigades readied for the counteroffensive have been sent to the front.

He stressed that any compromise with Russia to end the war would "destroy" Kyiv's statehood as Moscow "hates" Ukraine and seeks to recreate the Soviet Union.

"For us a compromise does not exist because Russia hates us, it came to destroy the concept of the Ukrainian state," Podolyak said.

"A compromise would in one way or another lead to the slow loss of Ukraine and its statehood" and the "return of the Soviet Union," he added.