EU foreign affairs chief Federica Mogherini lauded Iranian President Hassan Rouhani on his resounding re-election victory on Saturday and pledged the bloc would maintain work to implement the 2015 nuclear deal.

"Iranians passionately took part (in) the political life of their country. I congratulate President Rouhani for (the) strong mandate received," Mogherini said on her twitter account.

"The EU is ready to continue work for full JCPOA (nuclear deal) implementation, bilateral engagement, regional peace, and meet expectations of all people in Iran," she added.

The European Union and other top world powers reached agreement with Iran in 2015 on curbing its nuclear programme and they began easing economic sanctions in January 2016.

Brussels billed the agreement as clearing the way towards improved relations with Iran but progress has been slow and tensions have risen over Tehran's continued support for the ruling Assad regime in Syria who the EU believes must step down.

Rouhani, a 68-year-old moderate cleric who spearheaded the nuclear deal with world powers, won 57 percent of the vote, according to Iranian Interior Minister Abdolreza Rahmani Fazli.