Eight United States senators have sent a letter to President Joe Biden calling on him to offer Israel an ultimatum: expand aid to Gaza or lose US military assistance.

The letter, released on Tuesday, is the latest effort by US legislators to question ongoing US support for Israel amid its war in Gaza. It also comes as Biden himself has shown more willingness to openly criticise Israeli Prime Minister Benjamin Netanyahu.

Bernie Sanders and Elizabeth Warren, both former presidential candidates, were among the senators who signed the letter. Other signatories include Hawaii’s Mazie Hirono, Maryland’s Chris Van Hollen and Oregon’s Jeff Merkley.

They called on Biden to comply with Section 620I of the Foreign Assistance Act, which bars aid to countries that restrict access to humanitarian assistance.

“According to public reporting and your own statements, the Netanyahu government is in violation of this law,” the eight senators said in the letter.

“Given this reality, we urge you to make it clear to the Netanyahu government that failure to immediately and dramatically expand humanitarian access and facilitate safe aid deliveries throughout Gaza will lead to serious consequences, as specified under existing US law.”

Israel has denied blocking humanitarian assistance for Gaza, but international workers have accused Israeli authorities of making aid delivery difficult, through ongoing violence, closed border crossings and other impediments.

In February, for instance, United Nations workers accused Israel’s navy of firing on a convoy carrying food into northern Gaza. The Palestinian enclave has been under siege since October 7, with limited access to food, water and other basic supplies.

More than 31,180 Palestinians have died in Israel’s military campaign, and more are at risk of starvation and malnutrition, according to experts.

“The United States should not provide military assistance to any country that interferes with US humanitarian assistance,” the senators said in their letter.

“Federal law is clear, and given the urgency of the crisis in Gaza and the repeated refusal of Prime Minister Netanyahu to address US concerns on this issue, immediate action is necessary to secure a change in policy by his government.”

The UN has said regular ground deliveries equalling about 300 trucks a day are needed to meet the needs of Gaza’s population. Currently, a maximum of 150 are reaching the territory each day.

The Biden administration announced last week that it would set up a temporary pier off the coast of Gaza to deliver more aid by sea, although construction is expected to take several weeks. The US also recently began airdropping aid into the enclave.

Biden, meanwhile, has made contradictory statements about how he intends to approach the mounting concerns over Israel’s military actions.

On Saturday, for instance, he said that an invasion of the southern city of Rafah would be a “red line” for Israel not to cross. Still, he said he would never “leave Israel” nor “cut off all weapons” for the country.

Israel receives $3.8bn in military financing and missile defence assistance from the US every year, and the country enjoys widespread bipartisan support in the US Congress.

Still, a growing number of lawmakers, particularly on the left, have been willing to lodge criticism against the “ironclad” US ally.

Progressive legislators in the US House of Representatives, including Cori Bush and Rashida Tlaib, were among the first Congress members to call for a ceasefire in October last year.

Those calls have since been reflected in the Senate. In January, Senator Sanders introduced a resolution that called on the US State Department to produce a report within 30 days examining whether Israel has committed human rights violations in its campaign in Gaza.

The resolution, however, failed to pass the chamber, with a vote of 72 to 11 against it.

Still, pressure has been building to premise ongoing US aid to Israel on the condition that humanitarian law is complied with.

In February, Biden’s White House released a national security memorandum requiring countries that receive US weapons to provide written statements that they are acting within international law.

But that measure has fallen short of the leverage critics hope the US will exercise over Israel.

The Biden administration, meanwhile, has circumvented Congress to approve emergency arms transfers to Israel. It also continues to pursue more than $14bn in supplemental aid for the country.

In an interview on Al Jazeera’s Bottom Line programme on Saturday, Senator Van Hollen said it was time for the Biden administration to send a message to Israel: “If you continue to ignore us, there will be consequences.”