The European Union 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 Šefcovic, 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'.

The latest talks between the UK and the EU on the implementation of the Brexit Withdrawal Agreement in Northern Ireland have ended without agreement.

Mr Frost and Mr Šefcovic discussed the issues for three-and-a-half hours in London.

Mr Frost said there had been "no breakthroughs" with the EU on the operation of the Northern Ireland Protocol after "frank and honest" negotiations.

"The problem we've got is the protocol is being implemented in a way which is causing disruption in Northern Ireland and we had some pretty frank and honest discussions about that situation today," he said.

"There weren't any breakthroughs. There aren't any breakdowns either and we're going to carry on talking.

"What we really now need to do is very urgently find some solutions which support the Belfast Good Friday Agreement, support the peace process in Northern Ireland and allow things to return to normal."

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The Brexit minister maintained that the EU is insisting the protocol is implemented in a "extremely purist way" and said the UK will consider all available options.

He told reporters that "pragmatic solutions" must be found and did not rule out unilaterally extending the grace period on chilled meat exports between Britain and Northern Ireland.

"We don't see what risk is caused to Northern Ireland if chilled meats are imported there from GB.

"It's obviously best to find a negotiated agreement if we can and that’s what we are really intending to do. If we can’t, and we’re working very hard to do it, then obviously we consider all our options for next steps."

Pressed on the possibility of triggering Article 16 of the protocol, Mr Frost said: "There’s a range of things we may consider and we continue to consider them."

Last night, British Prime Minister Boris Johnson told European Commission President Ursula Von der Leyen that the UK was committed to finding practical solutions to the issues to protect the Good Friday Agreement.

He emphasised in a phone call the need for quick progress.

This evening, Mr Johnson said he was optimistic over talks with the European Union on smoothing trade with Northern Ireland, adding that the dispute would not overshadow the upcoming Group of Seven (G7) leaders summit.

"I think that what we want is something that enables us to protect trade flows, east/west, as well as north/south, and that's easily doable. I'm very very optimistic about this, I think that's easily doable," Johnson said.

Asked if the G7 summit would be overshadowed by a trade war with the EU, Johnson said: "I'm not worried about that."

Reaction in Northern Ireland

Reacting to today's meeting of EU and UK officials, DUP leader Edwin Poots called on the British government to "act unilaterally to protect Northern Ireland", if the EU is "deaf to the problems" surrounding the Northern Ireland Protocol.

In a statement, Mr Poots again reiterated calls for the Protocol to be removed and said that time for talking about the Protocol "has passed" and that it's "now time for action".

He said that he will be meeting Mr Šefcovic next week.

"If Brussels is deaf to the problems, then our own government must act unilaterally to protect Northern Ireland," Mr Poots said.

He described the protocol as a "disaster both politically and economically".

"The engagement between Lord Frost and Maros Šefcovic has been described as 'frank' but more talking is not what Northern Ireland needs. We need the NI Protocol to be removed", he said.

Mr Poots said the mechanism was "foisted" on Northern Ireland and claimed there are two-and-a-half times more checks at ports here than at Rotterdam.

"It has resulted in 15,000 checks per week meanwhile we have neither the infrastructure nor staff to meet the EU's demands."

He said the Protocol has failed and has only delivered instability and higher prices for Northern Ireland.

Earlier today, Sinn Féin's Michelle O'Neill said that the British government needs to "fulfil their international treaty obligations and work with the EU towards solutions".

Ms O'Neill said that longer term solutions to the issues around the Northern Ireland Protocol need to be found, but that these solutions will "not be found in unilateral action or short extensions of grace periods".

She said that she made it clear during today's meeting with Mr Šefcovic and David Frost that the DUP "do not speak for the north on Brexit or the protocol".

Ms O'Neill said that she made it clear that the Northern Ireland Protocol has allowed the "vast majority" of sectors in Northern Ireland to continue trading into both the British and EU markets and that "they want that to continue".

She said the Protocol is an "imperfect" consequence of Brexit but that it offers "very real protections" and gives Northern Ireland "a competitive economic advantage".

Biden to warn UK & EU not to imperil Good Friday Agreement

The US President Joe Biden will warn the UK and the EU not to imperil the Good Friday Agreement when he meets with leaders this week.

Mr Biden's National Security Adviser Jake Sullivan told the BBC that the US President has "deep" concerns that disagreements over the Northern Ireland Protocol could endanger the peace process.

Mr Sullivan said that he will tell fellow leaders at this week's G7 summit that the gains achieved by the Good Friday Agreement must be protected.

Mr Sullivan said that the president will urge the UK and EU to work out the specifics when it comes to the protocol and to find a way to proceed that works for both sides.

"But whatever way they find to proceed must at its core fundamentally protect the gains of the Good Friday Agreement and not imperil that," Mr Sullivan said.

On Monday, a group of prominent Irish Americans called on Joe Biden to 'stand up' for the Good Friday Agreement when he meets Mr Johnson this week.

The Ad Hoc Committee to Protect the Good Friday Agreement, which includes US political leaders and diplomats, urged Mr Biden to use the meeting as an opportunity to inform Mr Johnson that he will be appointing a US Special Envoy to Northern Ireland.

Former US Congressman James Walsh, who is a co-chair of the Ad Hoc Committee, called on the US president to demonstrate America's ongoing commitment to protecting the Good Friday Agreement.

"We are increasingly concerned that the Good Friday Agreement is becoming collateral damage as a result of Brexit and the ongoing disputes over the Northern Ireland Protocol," Mr Walsh said.

Additional reporting Brian O'Donovan, Laura Hogan