* Merger talks stopped, due to resume once Russian assets gone
* Shareholder meetings on merger postponed until May 2018
* CA Immo CEO says end of patience will be reached next year (Adds quotes, background on merger plans)
VIENNA, March 22 (Reuters) - Austrian property group CA Immo expressed frustration on Wednesday at the slow progress towards a planned merger with Immofinanz, calling on the other side to complete a precondition for the deal this year or risk its collapse.
CA Immo Chief Executive Frank Nickel said Immofinanz must shed its Russian assets in the fourth quarter at the latest for the two companies' merger talks, which are currently on hold, to be completed in time for shareholder meetings in May 2018.
In December, Immofinanz announced a delay in the sale or spin-off of its five Moscow shopping centres - a precondition for the merger both groups agreed on - to 2017.
"Our clients are in an uncertain situation (until May 2018). Sometimes we lose out in pitches because the vendor says 'How do I know what you will be called or if you will still want to do this later? We would rather sell to someone else'," Nickel said.
"We think the end of patience will be reached in 2018 and then (the merger) should really take place."
This month, Immofinanz, which bought a 26-percent stake in CA Immo in April, said it would soon invite around 25 parties that have expressed an interest to take part in the sale.
"This is something that one can put a timeline on only with difficulty. Honestly I would not want to have this task," Nickel said of Immofinanz's Russian efforts.
The companies plan to have shareholders vote on the deal in May 2018. When asked what would be the latest point at which Immofinanz would have to shed the Russian assets to meet that deadline, Nickel said the fourth quarter would be a "stretch, but it would still be possible."
Nickel added that the companies had not yet agreed on the future strategy of any merged entity or looked at one another's books. He also dismissed earlier estimates by Immofinanz of possible synergies, saying it was too early to tell. (Reporting By Shadia Nasralla; editing by Francois Murphy)