Since the 2016 British referendum vote for Brexit, negotiations have bounced back and forth between London and Brussels.
It is the first time the DUP has spoken since Mr Johnson met with the Taoiseach (Irish PM) Leo Varadkar on Thursday.
The two said in a joint statement they "agreed that they could see a pathway to a possible deal". "There's a way to go, it's important now our negotiators on both sides get into proper talks about how to sort this thing out".
"A no-deal Brexit will never be the choice of the EU", Tusk said.
But all eyes turned to a breakfast meeting in Brussels on Friday morning between British Brexit Secretary Steve Barclay and the EU's chief negotiator Michel Barnier.
The backstop is an insurance policy that was created to ensure no customs or regulatory checks on the border between Ireland and Northern Ireland after Brexit.
The two sides are tussling over the backstop agreement - an insurance policy championed by the EU to keep Northern Ireland in the bloc's customs union in order to prevent the creation of a physical border which could threaten peace on the island - which the United Kingdom argues is undemocratic.
Ms Foster said no barriers to trade could be erected within the UK.
Asked twice, he wouldn't promise that NI is leaving the EU customs union.
France's European affairs minister, Amelie de Montchalin, said early Friday that a no-deal Brexit "is probable, at this stage".
Following the breakfast meeting, Barnier began a briefing with ambassadors from the other 27 European Union capitals, seeking their input on entering political negotiations on a text.
"It's a tunnel with a very small light at the end of it", a second diplomat said.
Under no circumstances will we see anything that damages the whole of the United Kingdom to take full advantages of Brexit. The Prime Minister is very mindful of that. "Barnier will have to say whether we can or can't start negotiating a text", a European source told AFP. After meeting in northwest England on Thursday, Johnson and Varadkar said they had "agreed that they could see a pathway to a possible deal".
Nigel Farage, the leader of the Brexit Party, showed the challenge Johnson will face if he tries to move too far to accommodate Brussels.