KYIV, Ukraine/WASHINGTON Jan 30 (Reuters) - The United States will not provide the F-16 fighter jets that Ukraine has sought in its fight against Russia, President Joe Biden said on Monday, as Russian forces claimed a series of incremental gains in the country's east. 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. The city, where Russia's advance in the south was halted, had been under relentless bombardment until Ukraine pushed the front line back in November