Tusk made the comments on Thursday after speaking to Irish Prime Minister Leo Varadkar and also hours after Johnson said he had made a "genuine attempt to bridge the chasm" with EU officials before time runs out to reach a deal for the October 31 deadline for the UK to leave the bloc, the BBC reported.
Tusk tweeted: "Today I had two phone calls on #Brexit, first with Dublin then with London. My message to Taoiseach @LeoVaradkar: We stand fully behind Ireland. My message to PM @BorisJohnson: We remain open but still unconvinced."
Also earlier in the day, Varadkar said the new plans for the withdrawal agreement were welcome, but "fell short in a number of aspects".
The development comes as Johnson's Europe adviser, David Frost, was slated to hold another round of talks in Brussels in a bid to break the Brexit deadlock.
The EU's chief negotiator, Michel Barnier, has told European diplomats that he still had plenty of questions about the British proposal to replace the backstop - the measure designed to prevent a hard border on the island of Ireland after Brexit.
The new plans would keep Northern Ireland in the EU single market for goods but see it leave the customs union. But what happens to the border between Northern Ireland and Ireland remains a central sticking point, the BBC reported.
Downing Street has hoped that its new plan will replace the controversial Irish backstop provision that has proved the biggest obstacle to the existing withdrawal agreement.
The backstop was meant to keep a free-flowing border on the island of Ireland but critics - including Johnson - fear it could trap the UK in EU trading rules indefinitely.
The UK government has said that it was aiming to reach a final agreement at an EU summit on October 17.
( With inputs from IANS )