Trump's December declaration recognizing Jerusalem as Israel's capital and announcing plans to relocate the embassy there was met with worldwide condemnation. Last month Vice President Mike Pence told the Israeli Knesset in January that the move would be made in 2019. Critics argue that Trump was essentially circumventing Israel-Palestinian peace talks, where Jerusalem's status was to be one of the topics for negotiation.
While Israel claims Jerusalem as its "eternal and undivided" capital, Palestinians also claim the eastern part of the city as their capital in a future independent state.
The accelerated plan contradicts recent statements by Trump's secretary of state and vice-president.
The date was chosen to coincide with 70th anniversary of Israel's independence.
Lawyers at the State Department are looking into the legality of accepting private donations to cover some or all of the embassy costs, said the officials, who weren't authorized to discuss the issue publicly and demanded anonymity.
Meanwhile, the USA has started searching for a site as the permanent United States embassy to Israel, a process which Nauert described could be "a longer-term undertaking".
A ribbon-cutting ceremony is being planned for mid-May.
The move has been celebrated in Israel, which has long had its ministries, parliament and government offices in Jerusalem.
The rushed schedule means that at first the embassy will be located temporarily at the existing US consulate in the Arnona neighborhood in West Jerusalem.
For Trump, moving the embassy fulfilled a campaign promise that appealed to pro-Israel conservatives and evangelical voters.
"There is no greater gift than that!"
The embassy move is expected to complicate efforts to restart peace talks between the Israelis and Palestinians - and jeopardise the traditional, if disputed, United States role as an "honest broker" in efforts to resolve one of the world's most intractable conflicts.
Trump said at the time that he believed the move would push the peace process between them forward, since it would take Jerusalem off the table as a subject of disagreement.
Retrofitting existing diplomatic facilities is a cheaper option for the US, but there are now no consulates that match the capacity of the current embassy in Tel Aviv. Asked specifically about whether the security infrastructure would be adequate in that time frame, he said, simply, "We'll be fine".