The UK says it is ready to ignore the rules to prevent further disruption - sparking threats of EU retaliation.
The EU has warned Britain that it would react swiftly if it acted alone to change post-Brexit trading arrangements for Northern Ireland, but did not spell out what its steps might be.
"Trust, which should be at the heart of every partnership, needs to be restored," Maroš Šefčovič, the vice president of the European Commission overseeing EU-UK relations, told a news conference after a meeting with his British counterpart David Frost.
"If the UK were to take further unilateral action in the coming weeks, the EU will not be shy in reacting swiftly, firmly and resolutely to ensure that the UK abides by its international law obligations.
"Pacta sunt servanda," he added, using the Latin for 'agreements must be kept'.
However, Boris Johnson said a solution that protects trade flows and the peace process was "easily doable".
He said he was "optimistic" a solution could be found that "protects the peace process, but also guarantees the economic and territorial integrity of the whole United Kingdom".
Top EU official Maros Sefcovic said he was "positive we can find a solution" but added: "Our patience really is wearing very, very thin, and therefore we have to assess all options we have at our disposal."