"That's a first step but this step is not sufficient," European Commission chief Jean-Claude Juncker told reporters as he arrived for the second day of an EU summit in Brussels.
Belgian Prime Minister Charles Michel said the proposal outlined by May over an EU leaders' dinner on Thursday was "particularly vague".
The fate of around three million European citizens living in Britain after Brexit is one of the most contentious issues in the negotiations on Britain's withdrawal from the 28-member bloc, which began on Monday.
But she declined to say when that date would fall, offering only a window between March 29, 2017, when Britain triggered the Brexit process, and its expected departure two years later.
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The mood around the table was that May's offer was the "bare minimum", a European source told AFP, while Brussels is also clear that the cut-off date must be the day Britain leaves the EU.
"We don't want to buy a pig in a poke," Michel said, using an old-fashioned expression for agreeing to buy something without inspecting it beforehand. "The rights of European citizens should be guaranteed in the long term."
May has already set up a clash with Brussels by refusing to allow the European Court of Justice to arbitrate any disputes over citizens' rights in Britain.
German Chancellor Angela Merkel, the EU's most powerful leader, said late Thursday that May's plan was "a good start. But of course there are still many, many other questions".
"This is a fair and serious offer" that would give people certainty, she said, adding: "Of course, there will be details of this arrangement which will be part of the negotiation process."
May had previously refused to guarantee the rights of Europeans until the futures of one million British expatriates living in the rest of the EU were also secured, and she said her proposal depended on a reciprocal deal.
May called the election to secure a mandate for pulling Britain out of the EU's single market in order to cut immigration -- a key issue in the Brexit vote -- but some of her ministers are now warning that jobs and the economy must be the priority.
Juncker was asked if he knew what form of Brexit the government in London was now seeking, to which he replied: "No.
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