US officials said Thursday that they had authorized Ukraine to use US weapons to strike Russian forces amassed just across the border as they mount a major assault on the northeastern region around Kharkiv.

Asked if the United States could eventually relax rules further and let Ukraine strike deeper into Russia with US weapons, Blinken said that President Joe Biden's administration wanted Kyiv to be able to operate "deliberately and effectively".

"Over the past few weeks, Ukraine came to us and asked for the authorisation to use weapons that were provided to defend against this aggression," Blinken told reporters after NATO talks in Prague.

"That went right to the president," Blinken said.

"He's approved use of our weapons for that purpose. Going forward, we'll continue to be doing what we've been doing, which is, as necessary, adapt and adjust," Blinken said.

Russia launched a major offensive shortly after the US Congress approved a major new weapons package to Ukraine after a long delay.

Despite battlefield gains in recent weeks by Russia, Blinken voiced confidence that Ukraine would regain ground with the arrival of the weapons.

"Thanks to the extraordinary bravery of Ukrainians, and thanks to this enduring, strong support from our partners, the front in the east -- northeast -- is stabilising," he said.