European Commission chief, Jean-Claude Juncker told reporters as he arrived for the second day of an EU summit in Brussels.
"That's a first step but this step is not sufficient," Juncker told reporters as he arrived for the second day of an EU summit in Brussels.
British Prime Minister Theresa May outlined to EU leaders over dinner Thursday night her plans to grant permanent rights to an estimated three million European citizens living in Britain after Brexit.
It was her first offer on one of the most contentious issues of the negotiations on Britain's withdrawal from the European Union, which began Monday.
EU leaders have refused to debate the issue at the summit, saying it is a matter for the Brexit negotiators, but their public reaction Friday was distinctly cool.
Belgian Prime Minister Charles Michel said the proposal outlined by May was "particularly vague".
He said he looked forward to seeing the more technical details, including on judicial oversight of citizen rights, when Britain publishes a formal paper on the issue Monday.
"We don't want to buy a pig in a poke," he said, using an English expression for agreeing to buy something without inspecting it beforehand. "The rights of European citizens should be guaranteed in the long term."
The EU wants the European Court of Justice to arbitrate on any disputes over citizens' rights in Britain, a proposal London has rejected.
Austria's Chancellor Christian Kern said May's offer was a "good first proposal, which I appreciate, but it's clear that we have to invest much more work".
May promised that no EU citizen living in Britain when it leaves the bloc would have to leave, offering new permanent rights to those who arrive before a cut-off date, but without specifying when that date would fall.
"Of course, there will be details of this arrangement which will be part of the negotiation process," May said as she arrived at the summit on Friday.
But she added: "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.
"This is a fair and serious offer. I want to give those EU citizens in the UK certainty about the future of their lives, but I also want to see that certainty given to UK citizens who are living in the EU."