President Volodymyr Zelensky said Sunday that Ukraine will lose its war against Russia if the US Congress does not approve a major military aid package, as Russian forces heighten their pressure on a frontline city.

The Ukraine military acknowledged that the fighting around the frontline city of Chasiv Yar was "difficult" and "tense" but insisted its forces were resisting.

Amid heightened air attacks by both sides, Russia said one Ukrainian drone had hit the Zaporizhzhia nuclear plant that it has occupied since shortly after launching its invasion in 2022.

Ukraine has made increasingly urgent calls for the United States to start sending a proposed $60 billion aid package that has been blocked by Republicans in Congress for months.

"It is necessary to specifically tell Congress that if Congress does not help Ukraine, Ukraine will lose the war," Zelensky said during a video meeting.

It would be "difficult" for Ukraine to "stay" (survive) without the aid, Zelensky added. "If Ukraine loses the war, other states will be attacked."

Russia has stepped up attacks on Chasiv Yar as it seeks to build on territorial gains in recent months.

Chasiv Yar is less than 30 kilometres (18 miles) from Kramatorsk, an important rail and logistics hub for the Ukrainian army.

"The situation is difficult enough and tense," said Oleg Kalashnikov, spokesman for an army brigade deployed in the area. Russian was using infantry and air attacks, he told Ukrainian television.

"The Russians are trying to carry out assaults directly on the small towns of Bogdanivka and Ivanivske, outside Chasiv Yar."

Authorities said a Russian strike on Gulyaipole in the southern Zaporizhzhia region killed three people.

"Two men and a woman died under the rubble of their own private house, which was hit by a Russian shell," the head of the region, Ivan Fedorov, said on social media.

Officials said a woman was killed in an apartment block in Kupiansk, in the northeastern Kharkiv region that has also seen increased attacks in recent months.

Authorities in the main city of Kharkiv said Russia launched another attack on Sunday, wounding five civilians, a day after a deadly strike there.

Russia, meanwhile, said it had destroyed 15 Ukrainian drones over its  Belgorod and Bryansk border regions, reporting the death of one young woman.

Belgorod governor Vyacheslav Gladkov said shrapnel hit a car with a family of six inside in the village of Shagarovka, some 35 kilometres (20 miles) from the border with Ukraine. The woman died at the scene.

The Russian army said it had destroyed 12 Ukrainian drones over Belgorod region and three over Bryansk -- both territories regularly targeted by Ukraine.

Russia also said that a Ukraine drone had hit a dome on one of the six reactors at the Zaporizhzhia nuclear power plant that it occupies, but there was no radioactive release.

The International Atomic Energy Agency (IAEA) urged restraint, warning that "this is a serious incident with potential to undermine (the) integrity of the reactor's containment system".