European Union officials in Brussels have said a no-deal Brexit is "becoming increasingly likely".
They said they will "intensify their discussions" with the Irish Government over the coming days about a no-deal Brexit outcome.
The officials described the problem as "very fundamental and very complex", but stressed the talks will try to ensure any customs controls are "... away from border, if at all possible."
They added: "We are working with the Irish authorities so controls can be the least intrusive as possible."
The officials also said: "controls have to be where they belong, but that does not mean seeing visible infrastructure."
The comments were made at a media briefing on the EU's level of preparedness for dealing with a crash-out Brexit.
EU officials said a no-deal Brexit would result in "disruption to trade and supply chains" and their discussions with the Irish Government would include focusing on the extent of "financial help" from Brussels.
The officials maintained that a no-deal Brexit for the EU-27 Member States was "completely manageable", adding: "All States are confident they will have the necessary measures in place by March 29th."
In a warning to the UK, EU officials said even if there was a no-deal Brexit there would "still be a debt to be paid" and this would have to be dealt with before any future trade discussions could take place.
They also stressed that the UK had obligations under the Good Friday Agreement to ensure there was no return to the hard border of the past.
Officials also said they had "beefed-up" a helpline for citizens concerned about Brexit - the international freefone number is 00800 6789 1011.
In its statement, the European Commission said it had completed its preparations for a possible no-deal Brexit, but warned it would nonetheless cause "significant disruption for citizens and businesses".
If it crashes out without a deal on 12 April, the UK will not benefit from a transition period to new arrangements, but will immediately be subject to checks and tariffs on its exports to the EU, while "significant delays" can be expected at the borders, said officials.