Russia is building up forces near the northern part of Ukraine's Kharkiv region where it launched an offensive this month, but it still lacks the troop numbers to stage a major push in the area, Ukraine's top commander said Thursday.

Ukraine says it has stabilized the front in the northeastern Kharkiv region where Russian forces launched a cross-border assault on May 10 that opened a new front in the 27-month-old war and stretched Kyiv's outnumbered troops.

Col. Gen. Oleksandr Syrskyi said Russia was continuing to send additional regiments and brigades from other areas and from training grounds to bulk up its troops on two main lines of attack in Kharkiv region's north.

That includes the Strilecha-Lyptsi area between two small villages and the vicinity of the border town of Vovchansk where there has been street fighting.

"These forces are currently insufficient for a large-scale offensive and breakthrough of our defense," Syrskyi said in a statement on the Telegram messaging app.

Ukraine's General Staff said its forces were continuing to strengthen their defensive lines in the area and repelled one Russian attack near the village of Starytsia on the Kharkiv front on Thursday.

National Guard spokesperson Ruslan Muzychuk said Russian forces had not advanced in the Kharkiv region in the past day.

Syrskyi said Ukraine's "creation of an ammunition reserve" had also reduced the offensive capabilities of Russian forces.

The remark suggested Kyiv's acute shortages of artillery ammunition had eased since the United States finally approved a major aid package in April after months of delay.

U.S. Secretary of State Antony Blinken said on Wednesday that American weapons being delivered were helping to stabilize Ukrainian front lines.

Russia has concentrated most of its offensive pressure in Ukraine's east, where its troops have been able to make slow incremental advances since capturing the town of Avdiivka in Donetsk region in February.