The European Union must build on the momentum of the landmark nuclear deal with Iran and engage the Islamic republic on human rights, a MEP visiting Tehran said Tuesday.

EU Special Representative for Human Rights Stavros Lambrinidis could "discuss the terms of dialogue" about improving rights with the Iranian authorities, Belgian Green MEP Isabelle Durant told AFP.

Durant is part of an eight-member European Parliament delegation visiting Tehran. The lawmakers had said they would meet senior Iranian officials, as well as broaching human rights issues.

"Once the process has begun, it will be difficult to interrupt it," Durant said of the possible dialogue, stressing "the window of opportunity will not stay open for very long."

She was referring to the deal between Iran and world powers on November 24 to try to resolve a decade-long standoff over Tehran's controversial nuclear programme.

The West suspects Iran's nuclear drive was aimed at developing weapons, which Tehran has repeatedly denied.

Under the six-month interim agreement, Iran agreed to roll back parts of its nuclear activities in exchange for limited relief from punitive Western sanctions.

The United States, EU and United Nations also agreed to refrain from imposing new sanctions during the period.

But the US move to blacklist dozens of companies and individuals for violating existing sanctions against Iran had prompted Tehran to suspend talks.

Durant said that Iranian Foreign Minister Mohammad Javad Zarif at a meeting late on Monday had assured the delegation of Tehran's "commitment to the agreement, despite the American blacklisting move which has not facilitated the talks."

"We must indicate a posture of openness and dialogue to convince Iranian officials that discussion on human rights will be in the interest of Iran," Durant added.

She highlighted "severe violations of human rights such as the death penalty, arbitrary detentions and arrest of journalists," as well as the system banning the use of social media sites, which are used by a large part of population and even some government officials.

On Tuesday, the delegation met parliament speaker Ali Larijani and his brother, Mohammad Javad Larijani, head of the judiciary's high council on human rights.

The lawmakers have also held meetings with rights lawyer Nasrin Sotoudeh and filmmaker Jafar Panahi, who both won the Sakharov prize for human rights in 2012.

The meetings with Sotoudeh and Panahi has aggravated conservative camps in Iran, with MP Kazem Jalali saying the meeting was tantamount to "interference in Iran's internal affairs."

Sotoudeh was released from jail with a nearly dozen other political prisoners in September, part of a charm offensive by new President Hassan Rouhani.