Ukrainian forces have been on the defensive for months, while Russia's troops have steadily inched forward along the frontline.

On Monday Russia claimed to have captured a second village in as many days in eastern Ukraine, while a missile strike on the Black Sea port of Odesa killed at least four people and wounded 27.

Moscow has for weeks pressed its advantage at the front, an effort it has accelerated in recent days before crucial US weapons reach Kyiv's exhausted frontline forces.

"Ukraine has been outgunned for months, forced to ration its ammunition... But it's not too late for Ukraine to prevail," the NATO secretary general said at a press conference with Ukrainian President Volodymyr Zelensky.

He said months-long delays in US military aid to Ukraine had "serious consequences on the battlefield".

But Stoltenberg insisted that "more support is on the way".

"Our allies are looking into what more they can do and I expect new announcements soon. So we are working hard to meet Ukraine's urgent needs," he promised.

Ukraine has been dependent on Western military aid to counter Russia's larger and more powerful army.

Russia on Monday said it had captured the village of Semenivka near the strategic town of Avdiivka that was captured in February.

The announcement came a day after Moscow claimed the nearby village of Novobakhmutivka, with Kyiv saying the situation for its forces was worsening.

Semenivka is close to Ocheretyne, where fighting has intensified and part of which is reportedly under Russian control.

- 'We must disrupt Russia' -

Russia's strike on the strategically important port city of Odesa had left four people fighting for their lives, as well as killing four more, authorities said.

"Three women and a man were killed. According to the updated toll, 27 people were wounded, including two children... and a pregnant woman," Governor Oleg Kiper said on Telegram.

"Four of the wounded are in a serious condition, doctors are fighting to save their lives."

Russia regularly hits Odesa, which is crucial for Ukraine's exports.

Kiper earlier said the strike had damaged "residential buildings and civilian infrastructure".

Another Russian strike on Kharkiv, Ukraine's second city that is also a frequent target, wounded one person, local authorities said.

Regional governor Oleg Sinegubov said on Telegram that a 42-year-old man had been wounded by shrapnel and the blast following a Russian "missile attack".

Speaking alongside Stoltenberg, Zelensky urged faster weapons deliveries to bolster the front line.

"The Russian army is now trying to take advantage of the situation when we are expecting supplies from our partner," the Ukrainian leader said.

"That is why promptness in supply literally means stabilisation of the front line," he said.

"Together we must disrupt the Russian offensive," he said.

Speaking of the long-awaited aid, Zelensky said "some things have started to arrive" but declined to "go into details".

US President Joe Biden gave final approval for the aid -- blocked in Congress for months -- last week, saying shipments will start "right away".