Envoys from Britain, France, Germany, China, Russia and Iran will take part in the meeting.

VIENNA, Austria (AFP) — The remaining signatories to the faltering 2015 Iran nuclear deal will meet in Vienna on Friday with the survival of the landmark agreement at stake after Tehran vowed to continue to breach the deal’s limits on its nuclear program.

Envoys from Britain, France, Germany, China, Russia and Iran will take part in the meeting, which is the first time the six parties will have gathered in this format since July.

Since May, Iran has taken a series of measures, including stepping up uranium enrichment, in breach of the 2015 deal, with another such move likely in early January.

Iran insists that under the agreement it has the right to take these measures in retaliation for the US’s withdrawal from the deal in 2018 and reimposition of crippling sanctions.

Since last month, European members have in turn begun raising the possibility of triggering the so-called “dispute resolution mechanism” foreseen in the accord, which could lead to the resumption of UN sanctions on Iran.

On the eve of what was already likely to be a strained meeting, Britain, France and Germany accused Iran of developing nuclear-capable ballistic missiles, in a letter to the UN on Thursday.

Iranian Foreign Minister Javad Zarif dismissed the allegation as “desperate falsehood”.

However, despite the mounting tension observers say Britain, France and Germany are unlikely to trigger the dispute resolution mechanism on Friday when their diplomats attend the joint commission meeting chaired by senior EU official Helga-Maria Schmid.

Analysts say if UN sanctions are re-imposed and the deal falls apart, Iran could also withdraw from the Treaty on the Non-Proliferation of Nuclear Weapons (NPT).

“It’s not clear whether that’s worth the benefit,” Ali Vaez from the International Crisis Group told AFP.

But he warned the risk of the deal collapsing was increasing as Iran was “running out of measures that are easy to reverse and non-controversial.”

“Both sides are locked into an escalatory cycle that is just very hard to imagine that they would step away from,” he said.

Francois Nicoullaud, former French ambassador to Iran, also says tensions were expected to continue to rise.