The US may block more weapons systems to Israel if it goes ahead with a ground offensive in the southern Gaza city of Rafah, the US secretary of state, Antony Blinken, has said. The US has already suspended the shipment of 3,500 2,000lb (907kg) and 500lb (227kg) high-payload bombs following concerns over the scale of civilian casualties in Israel’s war in the territory. Blinken also said he was concerned that any further ground captured by Israel would create a vacuum “that’s likely to be filled by chaos, by anarchy, and ultimately by Hamas again”. Blinken’s remarks suggest Washington will not back down in the face of Israeli defiance over its plan to force the evacuation of Rafah, but it is an open question whether the US’s increasingly hardening tone will be enough to deter the Israeli prime minister, Benjamin Netanyahu. Speaking to CBS News, Blinken said the US president, Joe Biden, remained determined to help Israel defend itself, adding that the high-payload bombs were the only US weapons package currently being withheld. But that could change, he warned, if Israel launched a full-scale attack on Rafah. Biden has made clear to Israel that if it “launches this major military operation to Rafah, then there are certain systems that we’re not going to be supporting and supplying for that operation,” said Blinken. “We have real concerns about the way they’re used.” Israel needed, he said, to “have a clear, credible plan to protect civilians, which we haven’t seen”. He warned that Hamas was already seeing a resurgence due to the lack of a coherent, durable plan for Gaza. “We’ve been working for many, many weeks on developing credible plans for security, for governance, for rebuilding,” he said. “We haven’t seen that come from Israel.” The US national security adviser, Jake Sullivan, also warned that with more than 1 million civilians crowded in Rafah, “you would have really significant civilian casualties … Many Hamas folks would melt away because they are terrorists.” The president did not want to see US weapons used in that kind of operation, he said. There was further condemnation of a Rafah offensive from elsewhere, as the UN human rights high commissioner, Volker Türk, said it would breach international law. “I can see no way that the latest evacuation orders, much less a full assault, in an area with an extremely dense presence of civilians, can be reconciled with the binding requirements of international humanitarian law and with the two sets of binding provisional measures ordered by the International Court of Justice,” he said. The EU’s top diplomat, Josep Borrell, condemned the forced evacuation of Rafah as “intolerable”, while the Unwra chief, Philippe Lazzarini, said the claim of “safe zones” was “false and misleading.” The UK foreign secretary, David Cameron, refused to restrict British arms supplies at this stage even as he joined calls opposing a major offensive in Rafah. The six-week cycle under which arms sales to Israel are currently being considered by the Foreign Office suggests Cameron should be making a further recommendation later this week. Cameron would have an assessment in front of him that for the first time takes into account Israel’s killing of three British aid workers on 1 April. The Foreign Office refused to confirm this timetable, saying decisions were made on a rolling basis. Cameron said on Sunday that banning arms exports to Israel would help Hamas, but the Labour party set up a rare dividing line with the government on Gaza, saying that the UK should stop sending arms to Israel if it went ahead with the Rafah offensive, given the risk of a breach of international humanitarian law. The shadow paymaster general, Jonathan Ashworth, told Sky News that the UK should pause arms sales to Israel and said a full-scale offensive into Rafah would be “catastrophic beyond description”. Cameron said: “For there to be a major offensive in Rafah, there would have to be an absolutely clear plan about how you save lives, how you move people out the way, how you make sure they’re fed, you make sure that they have medicine and shelter and everything. We have seen no such plan … so we don’t support an offensive in that way.” He said he had made these points in a call with Ron Dermer, a close adviser to the Israeli prime minister, Benjamin Netanyahu, on Friday. Cameron also framed the imminent offensive in the context of Hamas failing to accept a ceasefire deal, saying the real pressure should be on the militant group. Hamas says it has accepted the deal, but Israel counters that the group altered the terms of the deal in unacceptable ways last week, including by making the ceasefire permanent. Cameron’s position of continuing to clear arms exports has been made more difficult by Biden’s administration publishing a report to Congress on Friday that said it was reasonable to assess that US weapons were being used by Israel to commit breaches of international humanitarian law. The US report did not propose a blanket ban on arms sales, but Biden said he was pausing the supply of some bombs and artillery as a result of attacks on civilian population centres. One source had predicted privately that a UK ban on the sale of offensive weapons had been imminent, but was subject to debate within Downing Street. Cameron sought to justify the UK taking a different decision from the US, saying: “The United States is a massive, bulk, state supplier of weapons to Israel, including 1,000lb [454kg] bombs and all the rest of it. The UK provides less than 1% of Israel’s weapons and it’s not a state supplier.” Cameron said he could give no updates on Hamas’s claim that the British-Israeli hostage Nadav Popplewell had died in Gaza. “Like everyone else, I watched the video on Twitter, X, last night, put out by Hamas of Nadav answering a question as to who he was,” he said. “And I watched that video and you just think, ‘What callous people they are to do that, to play with the family’s emotions in that way.’”