The General Assembly is set to vote on the Arab-backed measure on Wednesday after more than 120 Palestinians were killed by Israeli fire during protests near the border with Gaza since the end of March.
In a letter sent to fellow ambassadors, Haley said the United States had presented an amendment to the draft resolution that condemns Hamas for firing rockets at Israel and "inciting violence" along the border with Gaza.
The 193-nation assembly will vote on the US-drafted amendment before the vote on the resolution.
"Any resolution focused on the protection of civilians in Gaza must recognise the destabilising and reckless actions of Hamas, which endanger the lives and livelihoods of innocent civilians," Haley wrote in the letter seen by AFP.
The draft text condemns Israel's use of "excessive, disproportionate and indiscriminate force" against Palestinian civilians, but also deplores the firing of rockets by Palestinian militants into Israel.
However, it does not specifically mention Hamas. "This omission should be unacceptable to all member-states, given that Hamas fired over 100 rockets at Israel last month, provoked violent uprisings and obstructed the flow of humanitarian aid to the Palestinian people," wrote Haley.
"The proposed resolution is a fundamentally imbalanced text that ignores basic truths about the situation in Gaza." Diplomats expect the US amendment to fail and the Arab-backed resolution to be adopted, but it remains unclear how many votes it will garner in the face of strong US opposition.
The Palestinians are hoping to win as many votes as those cast in support of a resolution in December rejecting President Donald Trump's decision to move the US embassy to Jerusalem. That vote was 128 to 9, with 35 abstentions.
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