The EU needs an internal debate on reforms before the bloc can accept Ukraine - a major agricultural nation - as a member, European Council chief Charles Michel told AFP on Thursday.

Ukraine gained candidate status last year in a show of support after Russia sent its forces over the border.

"We must not wait until the last moment if the European Commission report in December recommends opening membership negotiations with Ukraine," Michel said as he was about to open an EU leaders' summit in Brussels.

"We have to say how we can finance and organise financial solidarity and avoid finding out later that we have a problem taking it in," he said.

Ukrainian President Volodymyr Zelensky, who was to take part in the start of the summit via video link, has called for EU membership talks to begin this year.

A decision on opening such negotiations requires unanimity from the European Union's 27 member states.

Nine other countries including Kosovo, Moldova, Serbia and Turkey, seek membership.

Candidates have duties to fulfil but "the 27 must not close their eyes and pretend not to see that they also have duties", Michel said.

He added he did not expect concrete decisions during the two-day summit on the issue but he wanted to launch a process and prepare for the next meetings.

Enlargement will be at the centre of future gatherings in Granada in Spain and in Brussels later this year.

Michel sent a letter last week to EU leaders inviting them to "reflect on the implications of enlargement" for the bloc.

"My role is to put in place a method. If the commission proposes to start negotiations, we must see what impact this will have on decision-making, common policies, funding," Michel said.

"We must establish our position before European elections" in 2024. "Europe must not fear debate," Michel said.

He added it was necessary to prepare because "enlargement will have an impact on agriculture, cohesion, the budget".

"We need an exact picture of the means available, how the money has been used, what changes are needed," Michel said.