Ukrainian President Volodymyr Zelensky told AFP in an exclusive interview he expects Russia to step up its offensive in the northeast and warned Kyiv only has a quarter of the air defences 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.

Zelensky said Russian troops managed to advance between five to 10 kilometres (3-6 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 on Friday in his first interview with foreign media since the offensive began.

Zelensky said the situation in the Kharkiv region has been "controlled" but "not stabilised".

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

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

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

- Kharkiv assault -

Russian President Vladimir Putin said during a trip to China this week the northeastern offensive was in retaliation for Ukraine's shelling of border regions and that Moscow was trying to create a "security zone".

Russian forces have taken 278 square kilometres (107 square miles) between May 9 and 15, their biggest gains since the end of 2022, AFP calculated using data from the Institute for the Study of War (ISW).

Russia said Saturday it had seized another village in the Kharkiv region.

"Units of the northern troop group liberated the village of Starytsya," the defence ministry said.

Ukraine's defence forces said they "were focusing their main efforts on preventing the Russian occupants from advancing."

Ukraine has evacuated almost 10,000 people from the northeast border area since Russia launched the assault.

Ukrainian officials have accused Russian soldiers in the eastern town of Vovchansk of capturing dozens of civilians to use as "human shields" to defend their command headquarters -- a claim AFP was not able to immediately verify.

- Mobilisation age lowered -

Putin said there was no intention at this stage to take Kharkiv, Ukraine's second city, about 30 kilometres (18 miles) from the border. More than one million people still live there.

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.

Zelensky acknowledged issues with staffing and "morale" within Ukraine's often outgunned and outmanned ranks, and signed a mobilisation law that came into force on Saturday.

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

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

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.

It also streamlined a long and bureaucratic mobilisation procedure, creating an online registration system where over 150,000 people had logged in on Saturday morning.

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

- 'Nonsense situation' -

Ukraine's strongest allies, the Baltic States and Poland, have grown nervous that Russia may try to attack them.

Poland announced it would spend $2.5 billion to fortify its eastern border, which includes Russia, Belarus and Ukraine.

As other 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.

He pushed on his allies to taker a firmer approach to Russia, including by allowing his armed forces to strike into Russian territory with Western weapons.

"We are in a nonsense situation where the West is afraid that Russia will lose the war. And it does not want Ukraine to lose it," Zelensky said.