Leading Republicans and Democrats have urged President Barack Obama to provide lethal defensive weapons to Ukraine in its fight against pro-Russian armed groups, BBC News reported.

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.

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 defense 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 U.S. politicians have been pressing Obama for some time to arm Ukraine, criticizing him for his indecisiveness.

The president has repeatedly stated he is studying such an option if diplomacy fails to end the crisis.