European countries involved in the Iran nuclear agreement met Tuesday to underline their support for the pact just hours before U.S. President Donald Trump was due to announce whether he would continue to abide by it.

Senior officials from Britain, France and Germany met in Brussels with Iran's Deputy Foreign Minister for Political Affairs, Abbas Araghchi, as the future of the landmark deal to curb the Islamic Republic's nuclear ambitions hangs in the balance.

In a statement, the Europeans said they "used this opportunity to reiterate their support to the continued full and effective implementation of the (agreement) by all sides."

They were not expected to meet again after Trump's announcement later Tuesday, but EU foreign policy chief Federica Mogherini, who helps supervise the deal's application and settle any disputes, was set for an evening of telephone diplomacy.

The exactly impact of Trump's decision will probably take some time to decipher. The agreement is endorsed by the UN Security Council and cannot be ended by any one party alone.

In the short term, should Trump lift his sanctions waiver, Congress has about 60 days to decide its next move. Iran can also trigger a dispute mechanism in the agreement, opening a maximum 45-day window for the airing of grievances and to seek a compromise. This could buy three months of valuable time.

Overall, Trump's threats have baffled the Europeans. They say the deal is working and note that the International Atomic Energy Agency has now certified 10 times that Iran is in compliance with its obligations.

They wonder how breaking this deal would improve things and fear it might only create a dangerous vacuum for Iran to resume its nuclear activities.

"If this agreement falls apart and you don't have a substitute, what do you gain? You make things worse because you probably trigger a nuclear arms race in the region," a senior EU official with close working knowledge of the agreement said Thursday. The official was not permitted to speak publicly about the Iran nuclear dossier.

The deal "is not based on assumptions of good faith or trust. It is based on concrete commitments, verification mechanisms and a very strict long-term monitoring done by the IAEA," the official said.

EU officials also warn that any changes Trump might want to make to this agreement would have to be done on the basis of "more for more." They believe Iran is unlikely to accept new constraints without concessions in return.

That said, it is still possible to ramp up pressure on Iran over its ballistic missile program or its destabilizing role in regional affairs through other sanctions, which would fall outside the scope of the agreement.

Just in case U.S. sanctions linked to this agreement do kick in, the EU is weighing how to protect the interests of European business working with Iran.

"We are having conversations obviously and we are working on a number of proposals that could protect European companies and operators," the senior official said, but declined to provide details.