"This is a difficult moment," she said after an extraordinary meeting of international donors in Brussels to discuss the Trump administration's decision to cut aid to the Palestinians.
"If I can put it in a headline, nothing without the United States, nothing with the United states alone," she told a press conference with the visiting foreign minister of Norway, a key donor.
The Palestinians have said the United States can no longer mediate in the Middle East conflict following Trump’s decision on Jerusalem, which they also consider to be their capital.
The row threatens to derail the Trump administration's peace plan, which had been set to be presented to both sides later this year.
The EU is the biggest donor to the Palestinians and has spoken out against Trump's Jerusalem decision.
"For the United States' plan, we'll wait and see. We don't have for the moment details or even a time framework, said Mogherini.
"For us what counts is first and foremost that everybody recognises the United States are essential for any process to realistically have a chance to succeed. But also for our American friends to understand that alone it will be more difficult to achieve anything."
Trump last week accused the Palestinians of disrespecting the US and threatened to withhold hundreds of millions of dollars in aid until they returned to the negotiating table.
Provoking Palestinian outrage, he reaffirmed his Jerusalem decision and said the disputed city had been taken "off the table", despite having previously said his recognition did not preclude later negotiations on its borders.
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