New | Chinese and Russian foreign ministers skip crunch Iran nuclear talks, which may miss deadline

The absence of the foreign ministers of Russia and China from weekend talks on Iran's nuclear programme dealt another blow to hopes the stalled negotiations will produce a deal by July 20.

The US - which is sending Secretary of State John Kerry to join three other ministers at the talks in Vienna as they enter their final week - is putting on a good face. State Department spokeswoman Marie Harf says the six powers talking with Iran remain "united in the negotiating room, as we always have".

But the absence of Russian Foreign Minister Sergey Lavrov is noteworthy, in light of suggestions by France that Moscow is deviating from joint negotiating stances with Iran. It may also reflect recognition that the two sides are too far apart, and the talks will have to be extended.

The most important disputes over how deeply Iran must cut its nuclear programme to gain sanctions relief are between Washington and Tehran, so Kerry's presence is important.

He will be able to talk directly to Iranian Foreign Minister Mohammad Javad Zarif.

British Foreign Secretary William Hague, French Foreign Minister Laurent Fabius and German Foreign Minister Frank-Walter Steinmeier are also attending. But the absence of Lavrov and Chinese Foreign Minister Wang Yi could be detrimental - it took foreign ministers or their deputies of all six nations to negotiate a preliminary deal with Tehran in November. China and Russia are sending lower-ranking officials instead.