US President-elect Donald Trump has urged Israel to "stay strong" until he assumes office next month.
In comments referring to last week's UN Security Council vote which criticised Israeli settlement building on occupied land, he complained Israel had been treated with "disdain and disrespect".
The comments came in his latest outburst on Twitter.
He tweeted the message ahead of a speech on the Israeli-Palestinian conflict by US Secretary of State John Kerry.
In it, he said Israel's future as a Jewish and democratic state was "in jeopardy" because of moves that damaged the prospect of a two-state solution to the Israeli-Palestinian conflict.
On Friday, the US chose not to veto a UN Security Council resolution calling for an end to Israeli settlement construction, leading to an angry response from Israel.
The issue of Jewish settlements is one of the most contentious between Israel and the Palestinians, who see them as an obstacle to peace and the creation of a viable Palestinian state.
More than 500,000 Jews live in about 140 settlements built since Israel's 1967 occupation of the West Bank and East Jerusalem. The settlements are considered illegal under international law, though Israel disputes this.
In two tweets issued on Wednesday morning New York time, Mr Trump said: "We cannot continue to let Israel be treated with such total disdain and disrespect.
"They used to have a great friend in the US, but ... not anymore. The beginning of the end was the horrible Iran deal, and now this (UN)! Stay strong Israel, January 20th is fast approaching!"
Israeli Prime Minister Benjamin Netanyahu replied on Twitter: "President-elect Trump, thank you for your warm friendship and your clear-cut support for Israel!"
Critics have urged the president-elect to use more conventional channels to communicate on international matters.
In his speech, Mr Kerry warned that current Israeli policy was putting a future peace agreement in jeopardy and was not in the best interests of Israel, the Palestinians - or the US.
"The two-state solution is the only way to achieve just and lasting peace between Israelis and Palestinians," he insisted.
"We cannot properly defend and protect Israel if we allow a viable two-state solution to be damaged before our own eyes."
Mr Kerry insisted the UN Security Council resolution passed on Friday was in accordance with US values - despite criticism from the Israeli government, which the secretary of state had an agenda "driven by its most extreme elements".
Mr Netanyahu hit back, saying Mr Kerry's speech was "obsessively focused" on settlements.
He said he was surprised Mr Kerry would focus one of his last speeches on Israel, while whole countries in the middle east collapse under terrorist attack.
Israel didn't need to be lectured by foreign leaders, he said.
The resolution stated that the establishment of settlements "has no legal validity and constitutes a flagrant violation under international law and a major obstacle to the achievement of the two-state solution and a just, lasting and comprehensive peace".
The US decision to abstain infuriated Mr Netanyahu, who has also taken diplomatic reprisals against the countries that voted in favour of the resolution.
Meanwhile, an Israeli committee has postponed a vote to authorise construction of almost 500 new homes in Jewish settlements in occupied East Jerusalem.
The move apparently follows a request from Mr Netanyahu's office.
Criminal investigation into Netanyahu
Meanwhile, this morning, an Israeli television channel reported that the attorney-general has ordered police to open a criminal investigation into two unspecified matters involving Mr Netanyahu.
Mr Netanyahu has in the past denied wrongdoing in the purchase of submarines from Germany, where media have reported a potential conflict of interest involving his lawyer.
The Channel 10 report said one of the two cases that the Attorney-General has been examining was not known to the public.