US senators from both parties renewed calls Thursday for President Barack Obama to boost military aid to Ukraine, while criticizing Europe's approach to the crisis.

The Obama administration has been under fire for months from some lawmakers who have criticized its failure to move decisively to provide Ukraine with "defensive lethal assistance" to defend itself against rebels backed by Russia.

John McCain, chairman of the Senate Armed Services Committee, told reporters such assistance "is not inconsistent with the search for a peaceful political solution."

McCain was joined by 11 other lawmakers, including some from the president's Democratic Party.

They spoke as French President Francois Hollande and German Chancellor Angela Merkel arrived in Kiev to push a new peace plan to halt an upsurge in deadly fighting in eastern Ukraine.

The lawmakers lashed out at Russia's President Vladimir Putin, and criticized European leaders for their ambivalence in dealing with the crisis.

"Putin understands nothing but force. He is a thug, he has not responded to sanctions (and) sanctions are not working," said Senator Richard Blumenthal, a Democrat.

McCain said he was hopeful that under German leadership the Europeans would come around, but added: "Frankly, I'm not overly optimistic as long as they are dependent on Russian energy."

"It's been a huge disappointment to me. Their actions recently have been reminiscent of the 1930s, but we will continue to hope," he said, alluding to the policies of appeasement that allowed the rise of Nazi Germany.

Senator Jack Reed said Washington should not act alone but "work with willing allies and partners to aid Ukraine."

Congress last December authorized $350 million in lethal and other weaponry to Ukraine including anti-artillery radar, surveillance drones and communications equipment.

But the decision to send the equipment rests with the executive branch.

Secretary of State John Kerry said Obama will "soon" decide whether to send arms to Ukraine to counter pro-Russian separatists, although the president prefers a diplomatic solution.

Senate Foreign Relations Committee chairman Bob Corker wrote the president Thursday urging him to fully implement the Ukraine arms legislation that he signed.

"We cannot allow Ukraine to fail," Corker wrote.

"The United States and our allies must be more invested in Ukraine's success than Russia is in its failure."