Sinn Fein leader Michelle O'Neill, who is set to become Northern Ireland's first minister, warned Monday she "will not be held to ransom" by Brexit disputes currently holding up the formation of a power-sharing government.
UK Northern Ireland Secretary Brandon Lewis has called on all parties in the province to form a government, after elections last week that saw the pro-Irish nationalist party win for the first time.
All five main political parties met Lewis for talks at the devolved legislature in Belfast, on their first day back on the job since Sinn Fein ended a century of dominance by pro-UK unionists in Northern Ireland.
But the pro-British Democratic Unionist Party, which came second, said post-Brexit trading arrangements need to be addressed first before it joins a new executive, in a move that could hold up its formation for months.
O'Neill said on Monday that "the DUP, but also the British government, must accept and respect the democratic outcome of this election".
The DUP collapsed the last power-sharing government in Belfast in February by withdrawing its first minister because of its opposition to the Northern Ireland Protocol.
The arrangement, signed as part the UK's exit from the European Union, provides sweeping checks on goods heading to Northern Ireland from the British mainland and keeps the province largely under European trading rules.
The DUP fears that by creating an effective border in the Irish Sea, Northern Ireland is being cast adrift from the rest of the UK and makes a united Ireland -- Sinn Fein's aim -- more likely.
London has repeatedly said it is prepared to trigger the Northern Ireland agreement's Article 16 suspension clause unless the deal it signed up to is changed -- a move the EU has warned could lead to a wider trade war.
"Brinkmanship will not be tolerated, where the north of Ireland becomes collateral damage in a game of chicken with the European Commission," said O'Neill.
"Make no mistake, we and our business community here will not be held to ransom."
UK minister Lewis earlier urged "a stable and accountable devolved government" and said all parties should "fulfil their responsibilities and form an executive as soon as possible".
"We have to address the outstanding issues relating to the Northern Ireland Protocol and we want to do that by agreement with the EU, but as we have always made clear, we will not shy away from taking further steps if necessary," he added.
DUP leader Jeffrey Donaldson said after meeting Lewis that "until we get decisive action taken by the UK government on the protocol we will not be nominating ministers to the executive."
He blamed the protocol for "driving up the cost of living, harming our economy, impeding the ability of businesses to trade with our biggest market and fundamentally undermining political stability".
Separate trading arrangements for Northern Ireland were agreed because the province has the UK's only land border with the EU.
Keeping the border open with neighbouring Ireland, an EU member, was mandated in the 1998 Good Friday Agreement that ended three decades of violence over British rule.
European Commission vice-president Maros Sefcovic urged London to "dial down the rhetoric, be honest about the deal they signed and agree to find solutions within its framework".
In Dublin, Irish prime minister Micheal Martin also urged the DUP to join the new executive and backed Brussels in its ongoing talks with London about the application of the protocol.
"I think the European Union has been flexible, has demonstrated flexibility, but every time up to now that the European Union has demonstrated flexibility, it hasn't been reciprocated," Martin told broadcaster RTE.
"I think the moment is now for both the EU and the UK... The British government wants to bring this to a conclusion."
