The United States will not provide the F-16 fighter jets that Ukraine has sought in its battle against Russia, US President Joe Biden said, as Russian forces claimed a series of incremental gains in the country's east.

Asked if the United States would provide the jets, Biden told reporters at the White House on Monday, ‘No.’

Ukraine planned to push for Western fourth-generation fighter jets such as the F-16 after securing supplies of main battle tanks last week, an adviser to Ukraine's defence minister said on Friday. ‘The next big hurdle will now be the fighter jets,’ Yuriy Sak, who advises Defence Minister Oleksiy Reznikov, told Reuters then.

Biden's comment came shortly after Ukrainian President Volodymyr Zelenskiy said Russia had begun exacting its revenge for Ukraine's resistance to its invasion with relentless attacks in the east.

Zelenskiy has warned for weeks that Moscow aims to step up its assault after about two months of virtual stalemate along the front line that stretches across the south and east. Ukraine won a huge boost last week when Germany and the United States announced plans to provide heavy tanks, ending weeks of diplomatic deadlock on the issue.

While there was no sign of a broader new Russian offensive, the administrator of Russian-controlled parts of Ukraine's eastern Donetsk province, Denis Pushilin, said Russian troops had secured a foothold in Vuhledar, a coal-mining town whose ruins have been a Ukrainian bastion since the outset of the war.

'Battle for every meter'

Pushilin said Ukrainian forces were continuing to throw reinforcements at Bakhmut, Maryinka and Vuhledar, three towns running from north to south just west of Donetsk city. The Russian state news agency TASS quoted him as saying Russian forces were making advances there, but ‘not clear-cut, that is, here there is a battle for literally every meter.’

Ukraine still controls Maryinka and Vuhledar, where Russian attacks were less intense on Monday, according to Ukrainian military analyst Oleh Zhdanov.

Pushilin's adviser, Yan Gagin, said fighters from Russian mercenary force Wagner had taken partial control of a supply road leading to Bakhmut, a city in Donetsk region that has been Moscow's main focus for months.

A day earlier, the head of Wagner said his fighters had secured Blahodatne, a village just north of Bakhmut.

Kyiv said it had repelled assaults on Blahodatne and Vuhledar, and Reuters could not independently verify the situations there. But the locations of the reported fighting indicated clear, though gradual, Russian gains.

Zelenskiy said Russian attacks in the east were relentless despite heavy casualties on the Russian side, casting the assaults as payback for Ukraine's success in pushing Russian forces back from the capital, northeast and south earlier in the conflict.

‘I think that Russia really wants its big revenge. I think they have (already) started it,’ Zelenskiy told reporters in the southern port city of Odesa.

Russian forces shelled a dozen settlements in the Bakhmut sector over the past 24 hours and trained tank, mortar and artillery fire on towns near Avdiivka to the south, the Ukrainian Army general staff said late Monday.

In central Zaporizhzhia region and in southern Kherson region, Russian forces shelled more than 40 settlements. In Kherson, the Russians shelled towns on the left bank of the Dnipro River that they abandoned in November, Kyiv said. Targets included the city of Kherson, where there were casualties.

Ukrainian forces repelled Russian attempts to advance on a number of towns and villages in Donetsk region — including Bakhmut and several nearby settlements, according to Ukrainian military analyst Oleh Zhdanov.

The front line in the sector has not changed, Zhdanov said.

The Russians also launched four rocket attacks on Ochakiv in southern Mykolaiv, the army said, on the day Zelenskiy met the Danish prime minister in Mykolaiv city, to the northeast.

Western delays

The hundreds of modern tanks and armoured vehicles pledged to Ukraine by Western countries in recent weeks for a counteroffensive to recapture territory are months away from delivery.

This leaves Kyiv to fight through the winter in what both sides have described as a meat grinder of relentless attritional warfare.

Moscow's Wagner mercenary force has sent thousands of convicts recruited from Russian prisons into battle around Bakhmut, buying time for Russia's regular military to reconstitute units with hundreds of thousands of reservists.

Zelenskiy is urging the West to hasten delivery of its promised weapons so Ukraine can go on the offensive.

Kremlin spokesman Dmitry Peskov said Western countries supplying arms leads ‘to NATO countries more and more becoming directly involved in the conflict - but it doesn't have the potential to change the course of events and will not do so.’

The US-based Institute for the Study of War think-tank said ‘the West's failure to provide the necessary materiel’ last year was the main reason Kyiv's advances had halted since November.

The researchers said in a report that Ukraine could still recapture territory once the promised weapons arrive.

The Russian invasion of Ukraine, which Moscow justifies as necessary to protect itself from its neighbour's ties with the West, has killed tens of thousands of people and driven millions from their homes.