Moscow warned on Monday that it would respond to Western supplies of long-range weapons to Ukraine by stepping up efforts to push Kyiv's forces further from its border.
"The more long-range weapons you supply, the further we will push away from our territory" the line of Ukrainian forces, Foreign Minister Sergei Lavrov told reporters.
Russian President Vladimir Putin has warned Moscow would hit new targets if the West supplies Ukraine with long-range missiles.
Washington and London have defied Mr. Putin's warnings against supplying Kyiv with advanced weapons, saying they would send long-range missile systems to Ukraine.
The U.S. last week announced it would give Kyiv its high mobility artillery rocket system, known as HIMARS, which can simultaneously launch multiple precision-guided missiles and is superior in range and precision to existing systems Ukraine has.
Britain said on Monday it will mirror the United States and send long-range missile systems to Ukraine too.
Ukrainian troops suffered setbacks after retaking parts of flashpoint eastern city Severodonetsk from Russian forces, local officials said on Monday, as the see-saw battle raged on for the strategically important city.
Thousands of civilians have been killed and millions forced to flee their homes since Mr. Putin ordered Russian troops into Ukraine on February 24.
Fighting since April has been concentrated in the east of the country, where Russian forces have made slow but steady advances after being beaten back from other parts of Ukraine, including the capital Kyiv.
"Fighting is very fierce in Severodonetsk," regional governor Sergiy Gaiday told Ukraine's 1+1 television.
"Our defenders managed to counter-attack and liberate half of the city, but the situation has worsened for us."
Russian forces "are destroying everything with their usual scorched earth tactics" so that "there's nothing left to defend", he said.
Mr. Gaiday said on Sunday that Kyiv's troops had "cleared half of Severodonetsk and are moving forward", after Ukrainian forces earlier appeared on the verge of being driven out of the city.
Artillery strikes have intensified on Severodonetsk and neighbouring city Lysychansk, where pensioner Oleksandr Lyakhovets said he had just enough time to save his cat before the flames engulfed his flat after it was hit by a Russian missile.
Lysychansk was among areas visited on Sunday by President Volodymyr Zelensky, who "got himself acquainted with the operational situation on the front line of defence", the presidency said.
In Moscow, Russian Foreign Minister Sergei Lavrov on Monday blasted European countries for blocking his plane from travelling to Serbia, saying: "The unthinkable has happened."
"This was a deprivation of a sovereign state of the right to carry out foreign policy," Mr. Lavrov told an online press conference in Moscow after several of Serbia's neighbours prevented his plane from passing through their airspace.
Mr. Lavrov had been due to hold talks with top officials in Belgrade, one of Moscow's few remaining allies in Europe since the launch of its military offensive in Ukraine.
Serbian daily Vecernje Novosti reported that Bulgaria, Macedonia and Montenegro had refused access to their airspace.
While Serbia has condemned Russia's military action in Ukraine, it has not joined the European Union in imposing sanctions in Moscow, despite its bid to join the bloc.
