Condoleezza Rice, the US Secretary of State said Washington was reserving judgment on the new government until it was formed.
"The Palestinian people are in desperate need of leaders to solve their daily problems and security"
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She denied in a television interview that the US had already decided to boycott the government.
Rice said prospects for Monday's summit between Mahmoud Abbas, the Palestinian president, and Ehud Olmert, the Israeli prime minister were "obviously more complicated" by the planned Hamas-Fatah coalition.
But diplomats and Palestinian officials said the US had also warned Abbas it would boycott all ministers in the new government, including non-Hamas members, unless its demands were met.
A White House spokesman, Tony Snow, reiterated the demands of the US and EU, saying that the new government must recognise Israel, renounce violence and agree to abide by previous peace agreements.
Haniya resigned, late on Thursday, only to be re-commissioned and charged by Abbas with forming a unity administration.
He has three weeks, with an extra two weeks' grace, to form and win parliamentary approval for a new cabinet.
Palestinians hope the new government will end infighting that has left 100 people dead since December and end a crippling Western aid boycott.
Under the terms of the Mecca agreement, agreed between Hamas and Fatah in the Saudi capital, Hamas will take 10 cabinet posts including Haniya, Fatah six, and others four.
The finance ministry will go to the Salam Fayad from the Third Way party, and the foreign ministry to Ziad Abu Amr, an independent MP, who was elected with Hamas backing.