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 defenses it needs to hold the front line.

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

Zelensky said Russian troops had managed to advance between five to 10 kilometers (three to six miles) along the northeastern border before being stopped by Ukrainian forces.

But he warned it may only 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 stabilized".

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 defense," 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 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 kilometers (107 square miles) between 9 and 15 May, their biggest gains since the end of 2022, AFP calculated using data from the Institute for the Study of War.

Russia said Saturday it had seized another village in the Kharkiv region, while Ukraine's defense forces said they "were focusing their main efforts on preventing the Russian occupants from advancing".

In his evening address Saturday, Zelensky said Moscow was paying a heavy price.

"The invaders are losing their infantry and equipment -– it's a significant loss," he said.

Like the initial invasion in 2022, Russia was "counting on a rapid advance through our land", he added.

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 using dozens of captured civilians as "human shields" to defend their command headquarters -- a claim AFP was not able to verify.

Two civilians -- aged 70 and 83 -- were killed when trying to leave Vovchansk by car, the Kharkiv regional prosecutor said.

"The battle in the area of Vovchansk is ongoing," Ukraine's armed defences said.

Mobilization age lowered

Putin said there was no intention at this stage to take Kharkiv, Ukraine's second city, about 30 kilometers 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 has acknowledged issues with staffing and "morale" within Ukraine's often outgunned and outnumbered ranks, signing a mobilization 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 the call-up.

The authorities have created an online registration system for anyone liable for military service to update their details. On Saturday morning, more than 150,000 people had logged on.

'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 pressed for a quick end to the war, Zelensky insisted Ukraine was 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 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.

But Russia said its air defenses on Saturday had destroyed French-made guided bombs and US-made anti-radar missiles over the western Belgorod region that borders Ukraine.