Britain, France, Germany, China, Russia and Iran will meet on June 28 as tensions in the Gulf soar after attacks on tankers and the shooting down of a US drone.

Iran's Islamic Revolutionary Guard Corps said Thursday it brought down an unmanned surveillance aircraft, prompting US President Donald Trump to warn it had made a "big mistake".

The European Union announced the emergency convening of the multi-country commission that oversees the nuclear deal, formally known as the Joint Comprehensive Plan of Action (JCPOA), as the bloc's 28 leaders met for a summit in Brussels.

"The meeting has been called with the intention of ensuring the continued implementation of the JCPOA in all its aspects and discuss ways to tackle challenges arising from the withdrawal and re-imposition of sanctions by the United States on Iran, as well as recent announcements by Iran regarding the implementation of its nuclear commitments," the bloc said in a statement.

On Monday, Tehran started a 10-day countdown to surpass an enriched uranium stockpile limit set by the deal, saying it would exceed the 300-kilogramme restriction by June 27.

The Islamic republic said it would only pull back from the threat if other parties to the nuclear deal "live up to their commitments".

Tensions between Tehran and Washington have escalated since Trump pulled out of the accord last year. The United States has since bolstered its military presence in the region and blacklisted Iran's Revolutionary Guards as a terrorist organisation.

The crisis in the Gulf reached new heights on Thursday with the downing of the US drone, which Tehran says infringed its airspace but which the Pentagon insists was in international skies.