"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.
Almost exactly one year after Britain voted to leave in the June 23 referendum, May promised that nobody would be forced to leave after Brexit, offering permanent rights over healthcare, education, welfare and pensions to Europeans who arrive before a cut-off date.
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.
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.
Publicly, leaders said they looked forward to seeing the more technical details when Britain publishes a formal paper on the issue on Monday.
"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".
No 'families split apart'
"I want to reassure all those EU citizens who are in the UK, who have made their lives and homes in the UK, that no one will have to leave, we won't be seeing families split apart," May said as she arrived for the summit on Friday.
"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.
But it was also probably intended as an olive branch as she struggles to maintain her authority after losing her parliamentary majority in a snap election two weeks ago, leaving her Conservative party struggling to form a stable government and throwing her entire Brexit strategy into doubt.
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."
'Much more work'
European leaders had warned there would be no debate at the summit on the plan for citizens' rights, as the Brexit negotiations are being conducted separately.
But both sides have expressed their desire to resolve the issue as quickly as possible.
"As citizens are the EU27's priority number one, it is a positive step that the UK will come with a detailed proposal on how to secure citizens' rights," an EU source said.
"It remains to be seen if the UK offer will be as generous as the one the EU27 set out in detail in the negotiating directives. At this stage there are, frankly speaking, some doubts."
Austrian Chancellor Christian Kern said: "It's a good first proposal, which I appreciate, but it's clear that we have to invest much more work."
May's proposal drew a derisory response at home, with a campaign group for EU nationals in Britain, called "the3million", declaring it "pathetic".
London Mayor Sadiq Khan, a member of the opposition Labour party, said it "does not come close to fully guaranteeing the rights of EU nationals living in the UK".
"It is unacceptable for the prime minister to be treating EU citizens living here and contributing to our economy and society as bargaining chips," he said.
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