LONDON (Reuters) - U.S. President-elect Joe Biden stressed the importance of protecting Northern Ireland’s peace deal in the Brexit process when he called UK Prime Minister Boris Johnson on Tuesday, hinting at potential tensions over Britain’s EU exit even as the pair emphasised common ground in other areas.Johnson’s government is seeking a trade deal with the EU but says it is willing to leave without one, which could complicate the situation at the sensitive Northern Irish border with Ireland - the UK’s only land border with the EU. The 1998 Good Friday peace deal that effectively ended Northern Ireland’s 30 years of sectarian violence created new institutions for cross-border cooperation on the island of Ireland. But Johnson has put forward legislation that would break the Northern Ireland protocol of the Brexit divorce treaty that seeks to avoid a physical customs border between the British province and EU-member Ireland. That prompted a warning two months ago from Biden, who has talked about the importance of his Irish heritage, that the UK must honour the 1998 agreement as it withdraws from the bloc or there can be no separate U.S. trade deal. “They talked about the importance of implementing Brexit in such a way that upholds the Good Friday Agreement,” a British official said after Tuesday’s Biden-Johnson call. “The PM assured the president-elect that would be the case.”