There is some evidence that Russia is bringing weapons into Ukraine to arm the pro-Russia rebels, U.S. officials said Friday. The development could prompt more sanctions to be levied against Russia by the European Union next week. 

A European diplomat in Washington, D.C., speaking on the condition anonymity, said the U.S. will also impose more sanctions once the EU completes a sanctions agreement against Russia next week, according to Reuters. 

The United States has welcomed the EU's new support for sanctions against Moscow, which are largely because of the downing of the Malaysia Airlines flight over pro-Russia separatist occupied territory that killed everyone on board. 

The EU drafted a preliminary plan to impose sanctions on Russia Friday, which included closing EU capital markets to state-owned Russian banks, restricting supplying dual-use and energy technologies and an embargo on arms sales to Moscow. 

"The European Union leaders have tied these tough measures not just to the cooperation on (investigating the Malaysian plane crash) but to an end of the arms flow across the border," a senior U.S. official told Reuters. 

The official added that Europeans know that arms are flowing into Ukraine from Russia. 

On Friday, the Pentagon said Russia was amassing troops along the Ukraine border once again, thereby helping the pro-Russia separatists by firing artillery into Ukraine. They added that the Russians are supplying rebels with large-caliber multiple-launch rocket systems.

Moscow has been angered by Ukraine's moves to build ties with Western Europe and has consistently denied supplying weapons to separatist forces. 

The United States is trying to stop Russia from aiding the rebels by imposing more sanctions and imposed the toughest round of sanctions last week. The sanctions target Rosneft, Russia's largest oil producer, Novatek, Russia's second-largest gas producer and Gazprombank, the country's third-largest bank. 

A Pentagon official said the U.S. is not finished imposing sanctions. 

Some critics of the Obama administration argue that the sanctions have not stopped Russian President Vladimir Putin from carrying out aggressive actions in Ukraine. However, the U.S. official said the sanctions are having some impact on Putin's decisions. 

A senior U.S. congressional aide said many U.S. officials want to impose more sanctions but that the White House favors tightening the financial sanctions, not imposing more. 

However, he said it is inevitable that the U.S. will impose more sanctions, since the European Union is now imposing sanctions as well. He said the sanctions will be more serious and will once more target Russian banks, defense firms and energy companies. 

U.S. State Department spokeswoman Marie Harf said Friday that the sanctions had been "incredibly effective against the economy."

While she admitted that Putin had not significantly changed his actions in Ukraine, she said the sanctions will begin working "the longer they are implemented".

Credit rating agency Fitch said that Russia has a "BBB" local currency issuer default rating because of the region's deteriorating security situation in east Ukraine and the sanctions imposed by the U.S. and EU

The agency said the shooting down of Malaysia flight MH17 in east Ukraine, which resulted in the deaths of 298 people, are expected to result in the levying of more sanctions against Russia. 

A European diplomat in Washington said the downing of the plane was a "game changer" for the EU

She said it looks likely that the EU will be imposing "something that looks and feels like sectoral sanctions" by the end of next week.