Brussels - The foreign ministers of Germany and Britain stressed the importance of Iran's nuclear deal on Thursday before meeting with Iranian Foreign Minister Javad Zarif in Brussels, one day ahead of a key sanctions decision expected from US President Donald Trump.

Trump has repeatedly called into question the 2015 nuclear agreement struck between Iran and six world powers - the United States, Britain, France, Germany, China and Russia - and has described it as "one of the worst" transactions ever entered into by the US.

EU: deal is a crucial agreement

He must decide on Friday whether to renew his country's waiver on nuclear-related sanctions against Iran, followed by a decision due Saturday on re-certification of the deal.

In contrast with the US position, the European Union is pushing for continued implementation of the agreement, which aims to prevent Iran from constructing nuclear weapons in return for the lifting of sanctions.

"For us Europeans, as neighbours of the region, it is a central component of our security," German Foreign Minister Sigmar Gabriel said ahead of Thursday's talks, which also included French Foreign Minister Jean-Yves Le Drian and EU foreign policy chief Federica Mogherini.

"The Iran nuclear deal is a crucial agreement that makes the world safer," added British Foreign Secretary Boris Johnson.

But the European ministers will also address Iran's "destabilizing regional activity," Johnson said, as well as its ballistic missile programme and a wave of political unrest that has gripped Iran in recent weeks.

Talks  overshadowed by US decision

"The right to peaceful demonstration within the law is central to any truly thriving society," Johnson said, while Gabriel called on Tehran to engage with the Iranian people on their "legitimate demands."

Protests began on December 28 over economic issues, but also focussed on Tehran's Middle East policies and the country's religious establishment as they spread to locations across the country. Eighteen demonstrators were killed and thousands of people have been arrested.

Critics have accused the EU of failing to quickly and unequivocally condemn Tehran's response to the protests, in an apparent bid to avoid undermining the nuclear deal.

Thursday's talks are likely to be overshadowed by any decision out of Washington on Friday.

Even if Trump does not reimpose nuclear sanctions on Iran, the US might soon decide on further measures in relation to other issues, such as support for terrorists abroad.

Such a move would not violate the terms of the Iran deal, but would run counter to its goal of easing tensions between Tehran and the international community.