KYIV -- The United States has confirmed to RFE/RL its delivery of American-made, Javelin antitank missile systems to Ukraine in a move that is welcome in Kyiv but will almost certainly enrage Moscow amid a four-year conflict that pits Russia-backed separatists against Ukrainian national troops.

"They have already been delivered," a U.S. State Department official confirmed on April 30 in response to an RFE/RL query on the handover of Javelins.

In a statement posted on Facebook, Ukrainian President Petro Poroshenko also confirmed the delivery and said his country continues "to strengthen our defense potential in order to repel Russian aggression."

"I am sincerely grateful for the fair decision of [U.S. President] Donald Trump in support of Ukraine, in defense of freedom and democracy," Poroshenko wrote. "Washington not only fulfilled our joint agreement, it demonstrated leadership and an important example."

Ukrainian Defense Minister Stepan Poltorak wrote on his Facebook page that Ukrainian troops would begin training with the new weapons on May 2.

A shipment of lethal aid would appear to deepen U.S. involvement in the simmering conflict and mark at least a symbolic victory for Ukraine in its effort to maintain Western backing in the ongoing conflict.

After months of heated debate in Washington and, reportedly, much reluctance on the part of U.S. President Donald Trump, the White House was said to have approved the Javelin sale in December.

That announcement sparked a sharp rebuke from Moscow, with Russian Deputy Foreign Minister Sergei Ryabkov accusing the United States of "fomenting a war."

Two sources who wished to remain anonymous as they were not authorized to speak publicly about it -- one in Ukraine and the other in the United States -- confirmed the Javelin deliveries to RFE/RL ahead of the State Department announcement.

Neither disclosed when the missile systems arrived in Ukraine, whether all the promised missiles and launchers had been sent or where they were being stored; or whether Ukraine’s military had begun training on Javelins. But one of the sources added that the Javelins were delivered “on time.”

The State Department provided no details beyond the confirmation of the delivery.

Ukrainian President Petro Poroshenko has lobbied hard to Western officials for more weapons, in addition to limited supplies of nonlethal aid from Washington and European allies so far and U.S. approval of commercial weapons sales.

A $47 million U.S. military-aid package approved last year and confirmed in March specified 210 Javelin antitank missiles and 37 Javelin launchers, two of them spares, for Kyiv.

State Department spokeswoman Heather Nauert said in December that U.S. military assistance to Ukraine was intended to bolster that country’s ability to "defend its sovereignty and territorial integrity, and to deter further aggression."

Kyiv and Western governments say Moscow has armed and coordinated Ukrainian separatists as well as provided Russian fighters to help wrest control of swaths of territory that border Russia since Moscow seized Crimea from Ukraine in March 2014.

The Javelins' delivery is likely to spur a response from Moscow, which rejects accusations of involvement despite mounting evidence that includes weapons movements and cross-border artillery barrages, captured Russian troops, and intercepted communications.

Responding to the approved delivery of the missiles to Kyiv in December, Russian Foreign Ministry spokeswoman Maria Zakharova said arming Ukraine would further inflame tensions between Moscow and Washington and push Ukraine "toward reckless new military decisions."

Since 2015, the U.S. has provided Ukraine with $750 million in nonlethal aid, including Humvees, night-vision equipment, and short-range radar systems.

There has been a recent uptick in fighting between Ukrainian soldiers and Russian-backed separatist forces, according to reports from the Ukrainian Defense Ministry and the Organization for Security and Cooperation in Europe’s Special Monitoring Mission (OSCE SMM).

A 3-year-old cease-fire deal known as Minsk II has helped to reduce the intensity of the fighting, but it has not ended the war.

The Defense Security Cooperation Agency said in March that while the Javelin sale would "contribute to the foreign policy and national security of the United States by improving the security of Ukraine" and "help Ukraine build its long-term defense capacity to defend its sovereignty and territorial integrity," it "will not alter the basic military balance in the region."