Senior US embassy diplomats in London, backed by the US national security adviser, Jake Sullivan, have directly warned the UK’s Brexit negotiator, Lord Frost, that he will inflame tensions in Northern Ireland if he does not compromise over border checks. A meeting between the US charge d’affaires Yael Lampert, currently America’s most senior diplomat in London, and Frost led to an urging by the US for Britain to come to a negotiated settlement with the EU, according to an internal UK government note. However, US officials questioned whether a formal demarche had been issued by the US to the UK, a rare reprimand between two such close allies. The US has said its role was to urge compromise from all sides. Sullivan told a briefing on Air Force One before Joe Biden landed in the UK on Wednesday evening: “Any steps that imperil or undermine the Good Friday agreement will not be welcomed by the US.” He declined to say if this is what he thought Boris Johnson was doing, adding: “The president was not issuing threats or ultimatum … [he] has been crystal clear about his rock solid belief in the Good Friday agreement as the foundation for peaceful coexistence in Northern Ireland. The agreement must be protected.” A senior US administration official said of the issue that “engagement wasn’t heightened or directed” by Biden. “As with any ally we have frank diplomatic conversations. Jake Sullivan spoke to these issues in his BBC interview, which addressed same themes we have been messaging privately.” Although the UK can insist it is seeking an amicable agreement with the EU, and its stance does not challenge the agreement, the US tone suggests the Biden administration is taking close interest in details of the talks, given it has a formal role to uphold the Good Friday deal. Biden’s affection for Ireland, where his ancestors are from, and scepticism about the wisdom of Brexit have hardly been disguised, but he has always accepted the British right to leave the EU. There were some indications in the UK government note that the US was prepared to dangle a free trade agreement as an incentive if the UK compromised over the legitimacy of border checks in Northern Ireland to protect the integrity of the single market. The US warnings came as direct talks between the EU and the UK over Northern Ireland appeared on the brink of collapse. London indicated it was still considering unilateral action to keep unhindered supplies flowing from Great Britain into the region. The talks are designed to rework the Northern Ireland protocol which set up a post-Brexit trade border between Northern Ireland and Great Britain. The European commission vice-president, Maroš Šefčovič, said patience was “wearing very, very thin” and described the relationship with the UK as “at a crossroads”. Amid fears that the escalating crisis over Northern Ireland would develop into a trade war, Frost said there had been “no breakthroughs” over the Brexit checks but no “breakdowns” either after a two-hour meeting with Šefčovič in London. They agreed to continue to try to find a solution before 30 June when a ban on chilled meats including sausages and mincemeat is due to come into force. On Wednesday evening, Johnson insisted that there was no crisis. “I’m very very optimistic about this. I think that’s easily doable,” he said, referring to an issue that was at the heart of the fraught Irish border negotiations two years ago: preventing a border on the island of Ireland while protecting trade within the UK after Brexit.