Leading Republicans and Democrats have urged President Barack Obama to provide lethal defensive weapons to Ukraine in its fight against pro-Russian rebels.
In a letter, House Speaker John Boehner and other lawmakers said Russia's actions in Ukraine were a "grotesque violation of international law".
They said that a recent truce had only consolidated rebel gains in the east.
Ukraine and the West accuse Russia of helping the rebels with weapons and soldiers - a claim Moscow denies.
Independent experts echo that accusation, but Russia insists that any Russians serving with the rebels are "volunteers".
At least 6,000 people are believed to have been killed since the conflict in Ukraine's eastern Donetsk and Luhansk regions erupted last April, a month after Russia annexed Ukraine's southern Crimea peninsula.
In other developments:
- EU foreign ministers are to meet later on Friday in Latvia to discuss the Ukraine crisis
- Nato's deputy chief Alexander Vershbow said on Thursday that Russia was less and less able to conceal the deaths of "large numbers" of Russian soldiers in eastern Ukraine
'Lack of clarity'
The bipartisan letter signed by 11 lawmakers said Russia's actions in Ukraine were "a challenge to the West and an assault on the international order established at such great cost in the wake of World War II."
"We urge you to quickly approve additional efforts to support Ukraine's efforts to defend its sovereign territory, including through the transfer of lethal defence weapons systems to the Ukrainian military."
The lawmakers said they believed this was the only way Ukraine could now "deter and defend against further aggression".
They also displayed impatience with the European Union, arguing there was a "lack of clarity" in the West's policy on the Ukraine issue and that President Obama should lead the way.
A number of US politicians have been pressing Mr Obama for some time to arm Ukraine, criticising him for his indecisiveness.
The president has repeatedly stated he is studying such an option if diplomacy fails to end the crisis.