WASHINGTON — House Speaker John Boehner and a group of top Democrats and Republicans stepped up the pressure on President Obama to provide lethal, defensive weapons to Ukraine in its fight against Russian-backed separatists.

In a letter to the president on Thursday, the 11 lawmakers wrote that Russia’s actions are more than an attack on Ukraine’s sovereignty, calling it a “grotesque violation of international law, a challenge to the West and an assault on the international order established at such great cost in the wake of World War II.”

The group said the so-called Minsk agreements, which led to last month’s cease-fire, have only consolidated Russian and separatists’ gains, and “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.”

Russian President Vladimir Putin denies arming rebels in the war in eastern Ukraine, which has killed more than 6,000 people and forced over a million to flee their homes. The fighting began in April, a month after Russia annexed the mostly Russian-speaking Crimean Peninsula.

Joining Boehner in signing the letter were Majority Leader Kevin McCarthy, R-Calif., and the senior Republicans and Democrats on the Foreign Affairs, Intelligence and Armed Services committees.