The US president told CNN that Washington would not provide bombs and artillery shells for the Israeli army to invade Rafah in southern Gaza.

But the images of charred bodies that emerged from an Israeli attack in Rafah on Sunday have raised questions about the credibility of Biden’s “red line”. An estimated 45 people were killed in the attack, which struck a cluster of tents sheltering displaced Palestinians.

“It is deeply disappointing to see President Biden continue to allow Israel to operate with impunity,” said Ahmad Abuznaid, director of the US Campaign for Palestinian Rights (USCPR).

“To issue a red line that you knew you weren’t going to follow through on not only means he will continue to be Genocide Joe, but it also just shows he’s weak politically.”

In recent weeks, Washington justified its failure to hold Israel to account by arguing that the offensive in Rafah was a “limited” operation, not the all-out assault that Biden had cautioned against.

US State Department spokesperson Matthew Miller reiterated that position on Tuesday, despite Sunday’s bloodshed and Israeli tanks pushing deeper into Rafah.

“We do not want to see major military operations take place there, the way that we saw them take place in Khan Younis and in Gaza City. At this point, we have not seen a military operation on the scale of those previous operations,” Miller said.

He added that the US could not “verify” that Israeli military vehicles are in central Rafah, which has been confirmed by Palestinian witnesses and Israeli media outlets.

A ‘meaningless’ red line

Palestinian rights advocates argue that the Biden administration is redefining what it considers to be an invasion of Rafah in order to be able to deny that an offensive is happening.

Yasmine Taeb, the legislative and political director for the advocacy group MPower Change Action, called Biden’s red line on Rafah “absolutely meaningless and merely a continuation of his callous and indefensible Gaza policy”.

“Israel is in violation of international humanitarian law, as well as US laws and policies, but nearly eight months of carnage in Gaza has apparently still not been enough for Biden to finally take a principled and consistent position by enforcing US laws and immediately suspending arms to Israel,” Taeb told Al Jazeera.

Israel followed Sunday’s bombing with another attack near Rafah on Tuesday that claimed the lives of at least 21 displaced Palestinians.

Mohamad Habehh, the director of development at American Muslims for Palestine, also called Biden’s “red line” meaningless.

“The Biden administration has failed to hold Israel accountable since October. We’re now in the eighth month of this. And we’re seeing that new massacre every single day,” Habehh told Al Jazeera.

Earlier this month, the US withheld a single shipment of heavy bombs to Israel, citing disagreements over Rafah. The move raised rights advocates’ hopes that Washington may finally be reconsidering its unconditional support for Israel.

That optimism soon dissipated after several US officials stressed “ironclad” support to Israel and the Biden administration approved the transfer of $1bn in weapons to its ally.

Israel receives at least $3.8bn in US military aid annually, and last month, Biden signed off on $14bn in additional assistance to the country.

The Center for International Policy (CIP), a US-based think tank, renewed calls for withholding weapons to Israel after the deadly attack on Sunday.

“The mass killing of civilians seeking refuge, whether by mistake or otherwise, is exactly what President Biden said would be unacceptable about an Israeli offensive in Rafah,” Dylan Williams, CIP’s vice president for government affairs, said in a statement.

“Biden shouldn’t wait for a pro forma Israeli investigation — he should stand by his word and halt arms right now.”

US says massacre ‘heartbreaking’

Israeli Prime Minister Benjamin Netanyahu has described the attack as a “tragic mistake” and promised an investigation.

At Tuesday’s State Department news briefing, Miller also called the massacre “heartbreaking”, but he attributed it to a “fire” rather than to Israeli bombardment. He said that Washington will be following the Israeli investigation closely.

But Habehh said citing Israel’s investigations is a tactic the US uses to deflect responsibility, allowing it to indefinitely delay making an assessment on rights violations.

Essentially, Habehh explained, that gives the US time to wait for stories of Israeli atrocities to “die out”.

“It’s very hard to allow the person who committed the crime to investigate whether or not they committed it,” Habehh told Al Jazeera.

As the horrors in Gaza intensify, advocates say it is becoming apparent that the Biden administration has no plans of changing course despite issuing statements against the invasion of Rafah and calling for the protection of civilians.

“Maybe Biden’s concerns for the Palestinians have floated away along with the port that they were building along the coast of Gaza,” Abuznaid said, referring to a marine hub Washington built to deliver aid to the territory, which was damaged by high tide in recent days.