Brussels is the final leg of a global diplomatic tour for Iran's foreign minister.
Mohammad Javad Zarif is meeting with his counterparts from the UK, France and Germany as he rallies support for the 2015 nuclear deal in the wake of the pull-out by the Trump administration.
What happened with the deal?
After years of talks, Iran agreed in July 2015 to freeze its nuclear programme in return for the lifting of punishing international sanctions.
However, last week UK President Donald Trump announced he was pulling out of the deal and reimposing sanctions.
What has Iran said?
That it is preparing to resume "industrial-scale" uranium enrichment "without any restrictions" unless Europe can provide solid guarantees that it can maintain trade ties despite renewed US sanctions.
Zarif embarked at once on a whirlwind global tour. He has visited Russia and China, the two other signatory nations, in a bid to bolster support.
On Monday, Zarif met with his Russian counterpart Sergei Lavrov. He had visited Beijing the day before.
After the talks, Zarif praised the "excellent cooperation" between Moscow and Tehran and said Lavrov had promised him to "defend and keep the agreement".
Lavrov, for his part, said Russia and Europe had a duty to "jointly defend their legal interests" in terms of the deal.
Russian President Vladimir Putin has already spoken with German Chancellor Angela Merkel and Turkey's President Recep Tayyip Erdogan about efforts to save the accord, after voicing his "deep concern" over Trump's decision.
And on Monday Putin met Yukiya Amano, the head of the International Atomic Energy Agency, telling him that Russia was "ready to continue to uphold the Iran nuclear deal despite the withdrawal of the United States".
What they are saying
"The final aim of these negotiations is to seek assurances that the interests of the Iranian nation will be defended," Iranian Foreign Minister Mohammed Javad Zarif.
"The agreement with Iran is working, we must do our utmost to preserve it," -- Maja Kocijancic, spokeswoman for the head of EU diplomacy Federica Mogherini.
"(European) cooperation with Russia, which until recently seemed impossible because of the Skripal (spy poisoning) case, with the expulsion of diplomats and the reduction of contact, is now receiving a fresh boost," - Andrei Baklitsky of the Moscow-based PIR Center nuclear safety NGO says the US decision has brought the other signatories together.