EU's Tusk says 'open but still unconvinced' by UK Brexit offer

British Prime Minister Boris Johnson has warned that if the bloc failed to listen to his new proposal it would share the blame for a likely chaotic divorce at the end of the month.

"Today I had two phone calls on Brexit, first with Dublin then with London," tweeted Tusk, who will play a pivotal role at a crunch Brexit summit set for October 17 in Brussels.

"My message to (Irish) Prime Minister Leo Varadkar: We stand fully behind Ireland. My message to Prime Minister Boris Johnson: We remain open but still unconvinced," said Tusk.

Tusk, a former Polish prime minister, coordinates EU summits and will guide strategy on Brexit at the summit after conferring with the EU's chief negotiator Michel Barnier.

The make-or-break summit will be held two weeks ahead of Britain's October 31 Brexit deadline.

Johnson must muster support for his fresh approach well ahead of the summit.

His offer focuses on a complicated proposal for preserving a free-flowing border between British-run Northern Ireland and EU member state Ireland after Brexit.