Brexit Secretary David Davis has paid a two-hour visit to the Irish border.
He was escorted by former senior police officer and Cooperation Ireland chief executive Peter Sheridan.
Mr Davis visited an autism centre in Middletown in Co Armagh as well as a nearby food processing company.
He also saw a former customs post between counties Armagh and Monaghan.
He tweeted: "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."