MOSCOW — Ukraine moved one step closer to receiving lethal military aid from the U.S. after the Senate passed a $612 billion defense policy bill late Thursday that would allocate $300 million of military aid, including radars and antitank weapons, to the war-stricken country.

Kiev has said it desperately needs the arms for its forces, which have seen disastrous losses in their fight against Russian-backed separatist rebels in Ukraine's eastern regions since the war began in April 2014.

The Senate must reconcile its bill with the House bill, and then it will be up to U.S. President Barack Obama to sign or veto it. Obama previously threatened to do the latter and has so far been reluctant to send lethal weapons to Ukraine, saying he believes in a diplomatic solution to the conflict that has cost the lives of more than 6,400 people.

A Russia-backed rebel tank moves to position, near Donetsk airport, eastern Ukraine, Friday, June 12, 2015. Heavy fighting continues at the frontline at the airport of Donetsk.

The president has several reasons to fret over the decision to supply Ukraine with arms, not least of which is the likelihood of Russia responding with a show of force — be it military exercises along its western border, or a fresh offensive under the guise of a rebel offensive in Ukraine's eastern Donetsk or Luhansk regions.

It would also officially mark America's entrance into a proxy war with Russia.

What's more, things like weapons have a way of disappearing in Ukraine, where corruption is still rampant more than a year after the Euromaidan revolution, according to a top Ukrainian security official, who asked to not be identified because he is forbidden from discussing the topic with journalists.

"We have seen the black arms market flourish since the start of the war in Donbass," the official told Mashable, using the colloquial term for eastern Ukraine. "Weapons can disappear all the time."

The official admitted it was also possible that some of the weapons could end up in enemy hands. Ukraine has lost several pieces of artillery and armor to the rebels, as well as an advanced radar system provided by the U.S. last year.

Another concern is putting lethal weapons in the hands of soldiers with little fighting experience. While U.S. Army paratroopers have begun training Ukrainian national guardsmen in a joint operation called "Fearless Guardian," most of the country's troops, including those who would be operating American weapons, have a mere few months or less of practical combat knowledge.

U.S. and Ukrainian soldiers at the opening ceremony of the Ukrainian-U.S. "Fearless Guardian" training operation in western Ukraine's Lviv region, Monday, April 20, 2015.

Critics fear that combination of deadly arms and inexperienced troops would spell disaster for a civilian population that has bore the brunt of artillery duels.

Before the Senate bill can go to Obama's desk, it must be reconciled with a version passed by the House — a process Arizona Sen. John McCain, who chairs the Senate Armed Services Committee and supports the bill, predicted could be finished in July.

Besides giving military aid to Ukraine, the bill prevents another round of base closures, and makes it harder for Obama to close the prison for terror suspects at Guantanamo Bay, Cuba.

"The Senate's overwhelming, bipartisan vote reflects the vital importance of this legislation to our men and women in uniform, especially at a time of growing threats to our national security. I hope today's result will encourage the president to abandon his misguided veto threat," McCain said.

The hawkish McCain, a longtime, staunch critic of Russian President Vladimir Putin, has pushed for the U.S. to give lethal aid to Ukrainian forces since last summer, and has made several trips to Ukraine to meet with top officials in a sign of solidarity with Kiev.

Ukr troops near Luhansk name a street after John McCain. I asked when Obama would get one. 'When he gives us weapons' pic.twitter.com/eU4cty75u4 — Christian Borys (@ItsBorys) June 19, 2015

In a significant measure taken to deter Russian military actions, the U.S. last week said it was set to approve a deal to store enough battle tanks, infantry fighting vehicles and weapons for 5,000 troops in several Baltic and Eastern European countries.

In response, Putin, who recently announced that Russia would add 40 new ballistic missiles to its nuclear arsenal this year, warned that stationing any military equipment along its borders could have "disastrous consequences."

Additional reporting by The Associated Press