19:17

The horse reference appeared to refer to Iranian perceptions that a package offered on Wednesday by the US had been withdrawn after a video conference on Wednesday night between the US negotiating team and Obama.

Diplomats said that some of the most heated arguments this week have been over the question of whether the UN arms embargo on Iran should be lifted along with economic and financial restrictions in return for accepting curb on its nuclear programme.

The debate split the six nation group negotiating with Iran, with Russia and China siding with Iran against the US, UK, France and Germany. “The arms embargo on Iran must be one of the first sanctions to be lifted … Especially since Iran is persistently supporting the fight against Isis [Islamic State] and the elimination of this threat in the region,” the Russian foreign minister Sergei Lavrov said, according to tweets from his ministry. Lavrov said he expected to return to the talks late on Thursday or early Friday.

Read Julian’s full dispatch here.