EU members agreed in principle on Wednesday to postpone the date of Brexit beyond the end of this month but did not set a new date, a European source said.
"The meeting was not meant to take any formal decisions and it didn't," the official said, after ambassadors from the 27 non-British member states met in Brussels.
"All agreed on the need for an extension to avoid a no-deal Brexit. The duration of an extension is still being discussed," he added.
The president of the European Council, Donald Tusk, has recommended EU leaders approve postponing the day of Britain's departure from the bloc from October 31 until January 31 next year.
But some capitals, principally Paris, would prefer a much shorter extension in order to help Prime Minister Boris Johnson quickly push a withdrawal agreement through the House of Commons.
If the member states can not agree, then an emergency EU summit could be called for Monday, but the EU source said the members had a "strong preference" for making a written decision this week.
The ambassadors plan meet again on Friday.
"In the meantime, President Tusk's consultations will continue," the official said.