Earnest said Washington has already provided military assistance worth some 100 million to 120 million dollars to Kiev in support of its battle against independence-seeking militants in Ukraine's east.

The fresh aid came as the administration slapped sanctions on more rebel leaders in eastern Ukraine in an effort to press for the implementation of peace deals.

Ceasefire agreements signed in September last year and in February this year in the Belarusian capital of Minsk have not been fully implemented, with sporadic fighting being reported daily in the eastern part of Ukraine.

"We do continue to have concerns about the commitment of the Russians and Russian-backed separatists to live up to the commitments that they made in Minsk, that there is still evidence that Russian military personnel are fighting alongside Russian- backed separatists," Earnest said.

"There continues to be evidence that Russia is continuing to transfer weapons and materiel across the border into Ukraine in support of separatists in eastern Ukraine," he added.

The Obama administration is mulling lethal aid to Kiev in case of the Minsk deals' failure to bring peace to eastern Ukraine, where at least 6,000 people have been killed in fighting that broke out in April last year.

Earnest said Obama "is very mindful of the potential risk that' s associated with providing additional lethal military assistance to the Ukrainians," including greater bloodshed in an escalated conflict.

The U.S. Department of Treasury on Wednesday targeted eight rebel leaders, a pro-rebel group and its three leaders, a Russian bank operating in Crimea, a Ukrainian region annexed by Russia in March last year, and three former officials who served under former Ukrainian President Viktor Yanukovych.