In a long-awaited statement on Iran that has been the subject of more than a week of wrangling in Brussels, the EU restated its commitment to saving the 2015 nuclear deal with Iran and took aim at Washington for abandoning the pact and reimposing sanctions.

The bloc hailed the creation last week by France, Britain and Germany of a system to allow firms to trade with Iran without falling foul of US sanctions as vital to supporting legitimate business and said the "resolve to complete this work is unwavering".

But with numerous European powers growing increasingly concerned about Tehran's missile programme, meddling in several Middle East conflicts and recent attempted attacks on opposition figures living in the EU, the bloc urged Iran to mend its ways.

The statement criticised Iran's "provision of military, financial and political support to non-state actors in countries such as Syria and Lebanon".

"The (EU) Council has serious concerns regarding Iran's military involvement and continuous presence of Iranian forces in Syria," the statement said.

Iran is a key supporter of Syrian President Bashar al-Assad and the statement urged Tehran to use its leverage to get Damascus behind UN-led efforts to end the civil war, which has claimed more than 360,000 lives since it began in 2011.

While defending the nuclear deal -- which limited Tehran's atomic ambitions in return for sanctions relief -- Europe has sought to keep up pressure with sanctions, most recently listing Iranian intelligence services over plots to assassinate regime opponents on Dutch, Danish and French soil.

Continuing Iranian missile tests have also alarmed the EU -- not to mention countries in the Middle East -- and Monday's statement called on Tehran to stop such activities.

"Iran continues to undertake efforts to increase the range and precision of its missiles, together with increasing the number of tests and operational launches. These activities deepen mistrust and contribute to regional instability," the statement said.

Brussels hopes the new Iran trading mechanism -- registered last week in Paris under the name INSTEX -- will keep Tehran in the nuclear deal by preserving some of the economic benefits it received.

Iran gave INSTEX a cautious welcome but US officials have dismissed the idea that the new entity would have any impact on efforts to exert economic pressure on Tehran.

Before INSTEX can go live, Iran has to set up a similar entity of its own so the two sides can clear trading on a barter basis without transferring money. It is not clear how long this will take.