"By the end of October, we will not have sufficient progress," Juncker said as he arrived for the EU summit in Tallinn. "I'm saying there will be no sufficient progress between now and October unless miracles will happen."

At the same time, he emphasized that progress is still being made in talks with the U.K.

British and EU negotiators stated during the fourth round of talks this week that "decisive steps forward" had been taken, but more work would be needed on both sides.

In the current stage, the EU wants to focus on the status of 3 million EU citizens living in the U.K. and that of British citizens living in the EU following Brexit, as well as the post-Brexit situation on the border of Ireland and Northern Ireland.

When sufficient progress has been made regarding these three issues, the block will be ready to discuss trade relations between the EU and the U.K. This will be decided at an EU summit in October.

Officials hope to conclude talks on Brexit terms by March 2019, when the U.K. formally leaves the EU.

Lithuanian president: Brexit talks may have to be extended

"Today we can say that negotiations are a little bit behind schedule and we are probably already facing the necessity of a transition or additional term of Brexit," Lithuanian President Dalia Grybauskaitė told reporters ahead of Friday's digital summit.

The fate of EU citizens in the U.K. is foremost in Lithuania, which has around 200,000 citizens currently living there.

Grybauskaitė said, however, that Lithuania would not stubbornly stick to its national interests, but rather seek a Brexit deal that is acceptable to all countries involved.