WASHINGTON Jan. 31 (Reuters) - The United States will not provide the F-16 fighter jets that Ukraine has sought in its battle against Russia, 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.