The Pentagon says Russian artillery is firing directly into Ukrainian territory from across the Russian border. REUTERS/Maks Levin

Russian military fired artillery rounds across the border into tumultuous east Ukraine on Thursday, a Pentagon spokesman said.

State Department spokeswoman Marie Harf told reporters that the U.S. has “evidence that Russia is firing artillery from within Russian to attack Ukrainian military positions.”

Army Col. Steve Warren, a Department of Defense (DoD) spokesman, called the artillery fire “a clear escalation” of the increasingly tense situation in the east. He said Russia has 12,000 troops staged near the Ukrainian border. That number has likely fluctuated significantly since Russia first moved troops there in March as opportunities for diplomatic solutions approached and passed. Russia claimed it moved troops and equipment away from the border in late May.

The U.S. and NATO have closely followed Russian military movements on the Ukrainian border since the armed conflict began earlier this year. NATO has used satellite imagery to support its assertion that Russia is actively supplying pro-Russian rebels, but Russia is quick to deny allegations.

The conflict in Ukraine spilled onto the international stage when Malaysia Airlines Flight MH17 was shot down by a BUK anti-aircraft missile over rebel-held territory in the east. All 298 passengers and crew aboard perished. Suspicions immediately fell on pro-Russian separatists over the downing, but rebel leaders deny any involvement. They say the Ukrainian military is responsible.

The Pentagon has maintained that Russia supplied pro-Russian separatists in the east with the BUK systems that took down MH17. Yesterday, the DoD referred to Russia as having a “possible role” in MH17’s downing, and accused Russia of supplying rebels with “more than 100 additional pieces of military equipment… in recent days,” including more anti-aircraft missile systems.

Ukrainian President Petro Poroshenko issued a unilateral cease-fire around the MH17 crash site earlier this week. Russia, the E.U., and the U.S. have all called for a full bilateral ceasefire following MH17's downing.