"If we want to conclude an agreement, decisions are needed now. This is still possible, but the political space to revive the JCPOA may narrow soon," he tweeted.

The JCPOA is the 2015 nuclear deal under which Iran committed to curbing its nuclear programme in return for lifting of international sanctions.

Former US president Donald Trump severely weakened the pact when he pulled America out of it in 2018, prompting Iran to drop its own compliance.

The Islamic Republic has now enriched uranium close to the level needed to build atomic weapons.

Efforts led by Borrell and his deputy Enrique Mora to get Iran and US back under the nuclear deal's terms have stalled, largely because of a late-running added demand by Tehran that Washington remove Iran's Revolutionary Guards from a terror blacklist.

Borrell in his tweet said he had again spoken with Iranian Foreign Minister Hossein Amir-Abdollahian, following up on talks he had during a surprise June 25 visit to Tehran.

Indirect negotiations between Iran and the US hosted by Qatar last week failed to produce a breakthrough.

The 2015 deal was strenuously negotiated by European powers France, Britain and Germany, along with Russia, China and the United States.

French President Emmanuel Macron said Tuesday he would "make every effort" to get Iran to revise its extra demands and take up the agreement worked out for a revival of the nuclear pact.