Michel Barnier has warned 'we are not there yet' on the three key issues
Efforts to reach a deal on the Irish border post-Brexit are set to intensify in the coming days.
The European Council President Donald Tusk is to fly into Dublin tomorrow to hold talks with Taoiseach Leo Varadkar.
It comes ahead of a crucial meeting between the British Prime Minister Theresa May and European Commission President Jean Claude Juncker in Brussels on Monday.
The meetings come as both sides work to make progress on the three core issues of the Irish border, the so-called 'divorce bill' and EU citizens' rights.
If EU leaders feel 'insufficient progress' has been made, they will not permit talks to move to the second phase - discussions on the future relationship between the EU and UK.
There are reports the UK government is close to finding a solution to the border issue, which could pave the way for trade talks to get underway.
It was reported earlier this week that there was a broad agreement on a framework for the UK to settle liabilities, with the final 'Brexit bill' expected to total around €45bn-55bn.
However, Theresa May and other leaders have denied a deal has been agreed.
However, Taoiseach Leo Varadkar yesterday warned that not enough progress has been made on the key issues in Brexit talks to move them forward.
Speaking in the Dáil, he said: "Much progress is being made on the financial settlement but it has not yet been agreed.
"None of the three items on which sufficient agreement is required before we can move to phase two have reached the necessary point.
"Progress is being made but insufficiently so at this stage."
The EU's chief Brexit negotiator, meanwhile, said that 'we are not there yet' in relation to the main negotiation issues.
Speaking in Berlin, Michel Barnier explained: "The next European Council will take place in 15 days' time. If real ‘sufficient progress' is actually made, the European Council will be able to open the discussion a possible transitional period.
"Then the Member States will define in 2018 the framework of this new partnership with the UK."
He added: "The negotiations on the United Kingdom's withdrawal are a complex task that we carry out with reason and determination, without aggression or naivety: ‘there is no place for Schadenfreude in Brexit'.
"There is neither revenge nor punishment in our mission."