By Julian E. Barnes and William Mauldin 

WASHINGTON--The U.S. believes Russia is firing artillery across its border at Ukrainian military positions, the State Department said on Thursday, an assertion that Moscow is playing a more direct role in the Ukrainian conflict.

The latest accusations show U.S. officials raising pressure on Moscow and more closely examining Russian activities near the rebel-held portions of eastern Ukraine since the downing of Malaysia Airlines Flight 17, which Washington blames on pro-Russian separatist fighters.

State Department spokeswoman Marie Harf also said the U.S. has evidence that Russia intends to deliver powerful rocket systems to the pro-Russia fighters in Ukraine, but declined to provide details about the systems or about how officials came to that conclusion.

"We have new evidence that the Russians intend to deliver heavier and more powerful multiple rocket launchers to the separatist forces in Ukraine and have evidence that Russia is firing artillery from within Russia to attack Ukrainian military positions," Ms. Harf told reporters at a news briefing, calling the findings "some pieces of info I've been able to get from our intelligence friends."

Russia has repeatedly denied supporting the separatist fighters. Moscow denounced as fantasy previous charges by Kiev that missiles fired from Russia brought down two Ukrainian military planes on Wednesday.

Defense officials said the U.S. still hasn't determined what brought down the two Ukrainian fighter planes. Rebels have claimed to have downed the plans with man-portable, shoulder-fired missiles, known as manpads, and U.S. officials have said that is possible.

Russia has been firing across the border into Ukraine for "several days, " according to a senior U.S. defense official. The U.S. has confirmed these strikes only within the last two days.

"For several days the Russians have been firing artillery into Ukraine," said Col. Steve Warren, a Pentagon spokesman. "This is clearly a military escalation."

The senior defense official said the U.S. doesn't have complete battle damage assessments, but said the strikes for the most part appear intended to harass Ukrainian forces.

Nonetheless, the official said the attacks show Russia's intent to provide broader support to the separatists. "It demonstrates direct Russian involvement in Ukraine," said the official. "It does up the ante."

Another defense official said the artillery fire began "about the same time" the Malaysian airliner was shot down. So far, the fire has all come from tube artillery and the U.S. doesn't have conclusive intelligence that Russia has fired anti-aircraft rockets into Ukraine.

Although used mainly to harass Ukrainian forces, U.S. officials remain worried that a sustained artillery barrage by Russian forces eventually could counter recent Ukrainian advances.

"Historically, artillery has been the biggest killer on the battlefield, " said the second defense official. "If used effectively, artillery can have a noticeable impact."

U.S. officials say the escalation by Russia is a risk and represents a sign that Moscow is worried about the Ukrainian government's counteroffensive against the separatists. Moscow, U.S. officials say, is worried that if the separatists don't turn back the Ukrainian government's advances, the separatist cause could be severely damaged or even lost.

Russia has between 15,000 and 18,000 troops on the Ukrainian border, but officials said there hasn't been a significant buildup of firepower since the downing of the Malaysia Airlines jetliner. Russian motorized infantry units all have artillery and rocket launchers assigned to them.

The strikes, officials said, are a sign that Russian President Vladimir Putin isn't going to back down in his support for the separatists, even in the wake of the shootdown of the Malaysian airliner.

Last week, the U.S. imposed a new type of sanctions targeting the financing activities of some of Russia's biggest companies, including Kremlin-controlled oil giant OAO Rosneft.

Since the Malaysia Airlines disaster, which killed scores of Dutch nationals and other Europeans, leaders in the European Union have also increasingly backed more serious sanctions against Moscow. EU officials on Thursday were considering sharp restrictions on Russian access to European financial markets and exports of militarily sensitive goods to Russia, and officials were also expected to impose sanctions on Russian individuals and entities.

Write to Julian E. Barnes at julian.barnes@wsj.com and William Mauldin at william.mauldin@wsj.com

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