WASHINGTON (AP) — At the heart of President Donald Trump's effort to get Ukraine's help investigating a political rival is nearly $400 million in critical military aid to help the country battle Russian-backed separatists.

That aid had widespread support until the Trump administration put the brakes on it this summer. In a July phone call, Trump pressed Ukrainian President Volodymyr Zelenskiy to "look into" Democratic presidential contender Joe Biden, shortly after the two men discussed potential military assistance for the country. The conversation has raised questions about whether Trump was using the aid as leverage to get help on the Biden issue.

Trump and Zelenskiy met on Wednesday at the United Nations, and in remarks to reporters they didn't discuss the specifics of the U.S. military aid. But Trump repeated his persistent complaint that other countries need to do more.

The amounts and types of military aid to Ukraine have long been points of contention inside the U.S. government, ever since Russia's 2014 annexation of the Crimean Peninsula, a move the U.S. and its allies view as illegal.

In the wake of the annexation, the Obama administration and national security officials clashed over whether to provide Ukraine with offensive weapons, weighing a desire to support the new pro-Western government and worries that it could trigger a wider war with Russia. Ukraine wants to join the NATO alliance, a move firmly opposed by Russia and not supported by some NATO members, due largely to similar concerns about inflaming Moscow.

Ukraine's hope of becoming part of the Western defense alliance is hindered by rampant corruption in government and business, and the five-year conflict with Russia-backed separatists in eastern Ukraine.

The Pentagon in June announced plans to send $250 million in aid to Ukraine, but its delivery was delayed. One defense official familiar with the Ukraine issue said the Trump administration told the Pentagon that it was not immediately releasing the aid to Ukraine because it was analyzing the extent to which Ukrainian was addressing long-standing U.S. concerns about corruption.

The official, who spoke on condition of anonymity to discuss internal deliberations, said the Pentagon's main concerns were whether Ukraine was acting on government corruption and whether other allies were also contributing aid. Those concerns mirror complaints that Trump has repeated in recent days.

Since then, Trump's call with Zelenskiy came to light. The U.S. aid was released two weeks ago.

The latest package of Defense Department and State Department assistance would provide a broad range of training and equipment. And officials familiar with the ongoing discussion said the bulk of the Pentagon aid would be spent by the end of the fiscal year that ends Sept. 30. The officials spoke on condition of anonymity to discuss details of the process.

Evelyn Farkas, an expert on the region who is a fellow with the Atlantic Council and a former Defense Department adviser, said providing military assistance to Ukraine is important.

If the U.S. doesn't help, she said in a tweet, "Russia gets away with its invasions & will eventually invade a NATO country. Then the US will have to go to war. It's cheaper and easier to help the Ukrainians now."

The $250 million in Pentagon funding will provide sniper rifles, rocket-propelled grenade launchers, counter-artillery radars, electronic warfare detection, secure communications, night vision capabilities and military medical aid.

The $141 million in State Department funding will include about $115 million for many of the same systems and equipment in the Pentagon package; $10 million for equipment, training and advisers to increase maritime awareness, improved secure communications and other equipment, and $16.5 million for maritime security in the Black Sea. That maritime aid would be aimed at identifying and tracking Russian ships and aircraft.