Kyiv, Ukraine - Ukrainian President Volodymyr Zelensky said he expects Russia to step up its offensive in the northeast and warned Kyiv only has a quarter of the air defenses it needs to hold the front line.

Russian forces, which made only moderate advances in recent months, launched a surprise assault in Kharkiv region on May 10 that has resulted in their biggest territorial gains in a year-and-a-half.

Ian exclusive interview with AFP, Zelensky said Russian troops managed to advance between three and six miles along the northeastern border before being stopped by Ukrainian forces, but added that the region could be the "first wave" in a wider offensive.

"I won't say it's a great success (for Russia) but we have to be sober and understand that they are going deeper into our territory," he said, speaking from Kyiv in his first interview with foreign media since the offensive began.

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Zelensky said the situation in Kharkiv region, from which thousands of residents have been evacuated, has been "controlled" but "not stabilized".

He doubled down on pleas to allies to send more air defense and fighter jets to combat Russia's air superiority as the war grinds through its third year.

"Today, we have about 25% of what we need to defend Ukraine. I'm talking about air defense," he said.

Ukraine needs "120 to 130" F-16 fighter jets or other advanced aircraft to achieve air "parity" with Russia, Zelensky said.

As he anticipates a widening Russian offensive, Zelensky acknowledged issues with staffing and "morale" within Ukraine's often out-gunned and out-manned ranks.

"We need to staff the reserves... A large number of (brigades) are empty," Zelensky told AFP.

With no end to the war in sight, Ukraine's army is struggling to recruit, while fighters are growing exhausted and angry at the lack of rotation.

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Many Ukrainian soldiers have been fighting for more than two years without the possibility of being discharged.

Under a controversial mobilization law that comes into force Saturday, Kyiv has lowered the age at which men can be drafted from 27 to 25 and tightened punishments for those who avoid being called up.

But lawmakers scrapped a proposal to grant soldiers who have served for more than 36 months the option to be discharged.

On Friday, Zelensky also signed a law allowing prisoners to be recruited in exchange for parole.

As Western allies press for a quick end to the war, Zelensky insisted Ukraine is still playing the long game.

"The West wants the war to end. Period. As soon as possible. And, for them, this is a fair peace," he said.

But for Ukraine, "final victory will lead to Russia's defeat".