The Biden administration is warning of policy changes if Israel doesn't take steps to better protect civilians and aid workers in Gaza amid international outrage over the killing of seven World Central Kitchen members.

President Joe Biden relayed the message in a phone call Thursday with Israeli Prime Minister Benjamin Netanyahu, the White House said.

The two leaders spoke for roughly 30 minutes, their first conversation since mid-March.

Biden "made clear the need for Israel to announce and implement a series of specific, concrete, and measurable steps to address civilian harm, humanitarian suffering, and the safety of aid workers," according to a White House readout of the call. "He made clear that U.S. policy with respect to Gaza will be determined by our assessment of Israel's immediate action on these steps."

While the warning is a significant shift in tone for the Biden administration, officials did not elaborate on what those policy changes would be if Israel doesn't comply and reiterated U.S. support for Israel's security.

Asked by ABC News Chief White House Correspondent Mary Bruce if President Biden is threatening to withhold aid to Israel if sufficient changes aren't made, National Security Council spokesman Kirby declined to get into any more detail.

"I'm not going to preview decisions that haven't been made yet," he said at another point in the White House briefing.

On timing, Kirby said they hope to see some announcement of changes on Israel's part in the "coming hours and days."

"What we are looking to see … is a dramatic increase in humanitarian assistance getting in, additional crossings opened up and a reduction in the violence against civilians and certainly aid workers," he said.

The warning comes amid growing frustration, and political pressures, between Biden and Netanyahu as Israel's war against Hamas terrorists in Gaza stretches into its sixth month.

Hamas carried out an incursion from Gaza into southern Israel by air, land and sea on Oct. 7, 2023, killing more than 1,200 people and taking 253 others hostage, according to Israeli authorities. More than 32,000 Palestinians have been killed and more than 74,000 others have been injured in Gaza since Oct. 7, amid Israel's ongoing ground operations and aerial bombardment of the strip, according to the Hamas-run Gaza Health Ministry.

World Central Kitchen has called for an independent investigation into the airstrikes, during which its convoy was struck after leaving a warehouse in central Gaza. The food charity said the vehicles were marked and their movements were coordinated with the IDF.

"This was a military attack that involved multiple strikes and targeted three WCK vehicles," the group said as it called on the U.S., Australia, Canada, Poland and the United Kingdom to join them in demanding a third-party probe into the events. The victims killed in the strike included citizens of those countries.

A senior adviser for Netanyahu told ABC News claims by the aid group that the strike was intentional were "absurd." Netanyahu has said Israel "deeply regrets the tragic incident."

Israeli Defense Minister Yoav Gallant told U.S. Defense Secretary Lloyd Austin on Thursday that a "thorough and transparent investigation is being conducted" and findings will be shared with partners, according to a readout of a call between the two leaders provided by Gallant's office.

While Kirby said the World Central Kitchen strikes prompted Biden to talk to Netanyahu on Thursday about his concerns, he said the two leaders didn't talk about the strikes or what exactly happened in "great detail."

Vice President Kamala Harris, Secretary of State Antony Blinken and national security adviser Jake Sullivan also dialed into Thursday's call between President Biden and Netanyahu, according to the White House.

Blinken, who is in Brussels to meet with NATO allies, told reporters Biden told Netanyahu the strikes that killed aid workers and the overall humanitarian situation in Gaza are "unacceptable."

In their conversation, Biden also "underscored" the need for an immediate cease-fire deal in order for hostages to be released and allow more humanitarian aid to get into the Gaza Strip, the White House said.

They also talked about Iranian threats against Israel, and Biden "made clear the United States strongly supports Israel in the face of those threats."

Kirby at the podium made clear U.S. support for Israel's ability to defend itself from a range of threats, not just Hamas, is "ironclad."

ABC News' Mary Bruce contributed to this report.