The $95 billion military aid package for Ukraine, stalled in Congress, could have weakened the country's front lines and could allow Russian forces to exploit those vulnerabilities to make "sudden and surprising advances" if the supply of equipment remains interrupted, an American-based non-profit said in a report.

"Ukrainian shortages of ammunition and other war materiel resulting from delays in the provision of US military assistance may be making the current Ukrainian front line more fragile than the relatively slow Russian advances in various sectors would indicate," the Institute for the Study of War said in an assessment of the two-year-old war released on Wednesday.

The report warned that "almost all Ukrainian units and formations have to husband ammunition and material because of the overall ammunition shortage and that some Ukrainian units with limited ammunition and materiel can only hold their current positions if Russian forces do not 'attack with full force.'"

Ukraine's prioritizing areas along the front that are most susceptible to Russia attack could create "vulnerabilities elsewhere," allowing Moscow to "make sudden and surprising advances if Ukrainian supplies continue to dwindle," it said.

The ISW report cited interviews by Ukrainian military officials in several media outlets describing how Russian forces "will be able to determine the time, location, and scale" of military operations at the front line where Ukrainian forces are facing shortages of arms.

"The current frontline is likely thus not stable, and timely Western resourcing of Ukrainian troops is essential to prevent Russia from identifying and exploiting an opportunity for a breakthrough on a vulnerable sector of the front," the ISW said.

The Senate approved a $95 billion aid package with $60 billion allocated for Ukraine in February, but the package has stalled in the House because of Republican opposition.

President Joe Biden earlier this week announced that the United States would send $300 million in new aid to Ukraine.