Pentagon spokesman Colonel Steve Warren said Friday that Russia had moved rocket systems and up to 12,000 troops near Ukraine. He also said that the country continued to fire across the border.

"For the last several days Russian forces using Russian artillery from Russian soil have conducted attacks against Ukrainian military positions in Ukraine," Warren said on Friday. "This is unquestionably an escalation from a military perspective."

US officials fear Russia could transfer the weapons to separatists, adding weapons to a conflict that has displaced more than 200,000 people in a matter of months.

On Friday, a Ukrainian security spokesman said that 13 soldiers were killed while battling separatists in the past 24 hours. The army also reported that its soldiers came under artillery fire from the Russian side of the border overnight.

Warren declined to say what impact the cross-border attacks had had on Ukraine's military. He added that US officials had no indication that Ukraine had fired back into Russia and had received no reports of civilian casualties.

Sanctions against Russia?

Officials from the 28 EU member states will meet on Tuesday to review proposals for new sanctions against Russia for its support of Ukraine's separatists. Calls for tighter measures by the bloc have grown even louder following the downing of a Malaysia Airlines jetliner in eastern Ukraine just over a week ago, allegedly shot out of the sky by the faction Russia supports.

In a news release on Friday, the European Union announced that the officials had also slapped EU-wide asset freezes and travel bans on 15 more Russians and allied Ukrainians. The measures are to take effect immediately and would bring the number of people under EU sanction over Russia's actions in Ukraine to 87.

White House press secretary Josh Earnest said the recent escalation had also prompted officials in the United States to discuss possible new sanctions against Russia.

"It only underscores the concerns that the US and the international community has about Russian behavior and the need for the Putin regime to change their strategy," Earnest said on Friday.

mkg/tj (Reuters, AFP)