Britain's Brexit Secretary David Davis has made a two-hour visit to the border area.
Mr Davis, who has a key role in negotiating the terms of the UK's exit from the European Union, has previously said he had never seen the border on the island of Ireland at first hand.
He was accompanied by Peter Sheridan, a former senior officer in the PSNI and now the chief executive of the Co-operation Ireland charity.
The visit, which was not announced to the media beforehand, took two hours.
It is understood Mr David visited an autism centre that is funded by both governments in the Middletown area and also a food processing company.
He also saw the redundant customs post between counties Armagh and Monaghan and was shown how fields, hills, roads and lanes form the porous frontier.
In a tweet Mr Davis said: "Today I started what promises to be a busy week in Northern Ireland. As we leave the EU it's essential both the UK and EU do what it takes to keep the border, which I saw this morning, free from physical infrastructure. We are determined to get this agreed by October."
Today I started what promises to be a busy week in Northern Ireland. As we leave the EU it's essential both the UK and EU do what it takes to keep the border, which I saw this morning, free from physical infrastructure. We are determined to get this agreed by October. pic.twitter.com/W4zSuRSGRG— David Davis (@DavidDavisMP) April 23, 2018
British Prime Minister Theresa May has said her government was sticking to its proposals to ease trade with the European Union after Brexit, rejecting suggestions that plans to quit its customs union were again dividing her party.
A British government defeat in Britain's upper house of parliament last week has again raised questions over Mrs May's commitment to leaving the customs union, a plan that has split not only her own party but also across the parliamentary divide.
The British government has proposed two ways of ensuring that trade is "as frictionless as possible" with the EU once Britain ends a status quo transition phase after it leaves in March next year.
The EU again rejected one of those last week.
"We've put forward proposals that will deliver a frictionless border and enable us to do trade deals around the rest of the world," Mrs May told reporters.
"I think that will be the best position for the United Kingdom and that's what we're working for," she added.
Earlier, a spokesman for Mrs May said the government had set out the options - a customs partnership or a streamlined customs arrangement - almost a year ago, and that it has "been moving forwards as one with the process of leaving the European Union".
Mrs May is facing a week of fresh parliamentary battles over Brexit as clashes centred on staying in a customs union with the EU continue to dog her government.
With flagship Brexit legislation vulnerable to further attacks in the House of Lords, pro-European Tory MPs are set to voice their concerns over withdrawal on Thursday.
Downing Street sources insisted there would be no backsliding over quitting the customs union with Brussels after reports that Mrs May was ready to take a softer line.
Though the looming Commons vote on a pro-customs union motion would be a symbolic, non-binding one, it has the potential to deepen Tory wounds on Brexit.
Mrs May is set to face calls from leading Brexiteers Boris Johnson, David Davis and Liam Fox to abandon her preferred form of a customs deal with the EU, according to The Times.
A showdown is expected to come at a meeting of the Cabinet Brexit committee scheduled for Wednesday.
The trio will tell the prime minister that a so-called customs partnership, where the UK would collect EU import tariffs on behalf of Brussels, would be unworkable, the report said.
Former Cabinet minister John Whittingdale told BBC Radio 4's The Westminster Hour that defeats like the one the British government suffered last week in the House of Lords over the customs union played into the EU's hands.
He said: "Part of the problem is that there are some in the European Union and in the Irish Republic who want to make out that we have to stay in the customs union and will not examine other alternatives.
"And votes like ones in the House of Lords are going to add to the view over there that somehow the British government might U-turn or go back.
"We need to make it absolutely clear that leaving the customs union is an absolutely fundamental part of Brexit. We then have to find a solution to the Northern Ireland border question."
What is the EU customs union?
Labour MP Chuka Umunna and backbench pro-Remain Tory Anna Soubry have tabled amendments to the Trade and Customs Bill, due to be debated on Thursday, that will make staying in the customs union a legal objective of the Government.
Mr Umunna said on Twitter: "The Brextremists threaten May with a leadership election if she concedes on the customs union but a new Tory leader would face the same parliamentary arithmetic."
Cabinet minister Sajid Javid tweeted that the referendum gave "clear instructions" to leave the customs union and accused some of seeing it as a "kind of post-Brexit comfort blanket".
The minister's comments were rebuked by the head of the CBI, Paul Drechsler, who tweeted in reply: "An MP of your talent should rise above ideology and lead based of fact, analysis and evidence - all of which favours a customs union. Always happy to discuss."
The Lords could deliver new defeats on the Government regarding incorporating the EU charter of fundamental rights into UK law, and pressing to ensure MPs get to decide what happens next if the Commons rejects a final Brexit deal with Brussels.
Additional reporting: Reuters, PA