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The White House has not seen “hard, tangible evidence” that Iran was directly involved in the Hamas attacks against Israel, but Tehran is nonetheless complicit, National Security spokesman John Kirby said on Monday.
Iran earlier denied any involvement in the Hamas attacks, saying the Palestinians were solely responsible for their actions.
"There's a degree of complicity here writ large," Mr Kirby said. "Nobody's walking away from the fact that Iran has long supported Hamas."
"That said, we haven't … seen any hard, tangible evidence that Iran was directly involved in participating in, or resourcing or planning, these sets of complex attacks that Hamas pulled off over the weekend."
He said Israeli officials claimed they had yet to find a "smoking gun", but that the US was going to keep looking to determine if there was a connection.
At the UN, Iran released a statement saying: "Iran is not involved in Palestine's response, as these actions are solely determined by the Palestinians themselves."
Tehran said the Palestinians had the freedom to make choices they considered to be in their best interests, and “to exercise their own judgment in determining their course of action".
But Israel's Foreign Ministry accused Iran of spreading “bloodshed” and directly arming Hamas.
"Hamas would not have been able to carry this murderous attack without the financing, logistical support and weapons it received from Iran,” the Israeli statement said.
Tehran, it read, bears direct responsibility for Hamas's capabilities, and therefore bears responsibility also for the use Hamas made of them
"The terrorist organisations involved are branches of the Ayatollah regime in Iran, which proactively promotes terrorist activity in Israel and against Israeli and Jewish targets around the world."
Israel also urged the international community to warn Syria and Lebanon against entering the conflict.
Tehran, which has made the Palestinian cause the central focus of its foreign policy, said it supports Hamas's decisions and measures in Gaza.
It said they represent a “completely legitimate response to seven decades of oppressive occupation and the numerous atrocities committed by the Israeli regime”.
The statement also criticised the international community and the UN for their inadequate response to Israel's “grave and serious violations” against the Palestinian population.
“This egregious violation of international law carries significant repercussions and necessitates that the international community and United Nations member states assume responsibility and act decisively to terminate this illegal occupation,” it said.
“Mere sympathy is no longer adequate.”
Iran claims the continuing situation in the occupied Palestinian territories stem from the “prolonged denial” of Palestinians' right to self-determination, and continuing Israeli aggression.
With additional reporting from Holly Johnston