DUBAI/RIYADH, May 11 - A UAE-based firm threw into doubt on Monday plans to develop a $5 billion aluminium smelter in Saudi Arabia, dealing the second blow in less than six months to the kingdom's industry ambitions. An executive of state-owned smelter Dubai Aluminium Co said the future of an aluminium plant project at Saudi Arabia's King Abdullah Economic City was "uncertain" in the current economic climate. Dubal held talks with Saudi Arabian Mining Co <1211.SE> in April over the plan, 13 months after it signed an initial agreement with the Saudi Arabian General Investment Authority and Emaar Economic City <4220.SE> to develop the 700,000 tonne per year smelter. The plant, to be built in King Abdullah Economic City -- the largest of four business and industry hubs under construction in the kingdom -- was hailed as a vote of confidence in the world's top oil exporter efforts to diversify its economy. "Plans for the King Abdullah Economic City plant are uncertain because of the economic conditions," Walid al-Attar, Dubal's vice president for marketing and sales, said in Dubai. Attar was not more specific, while Ma'aden said no deal had been reached yet. "A number of companies including Dubal have expressed an interest in applying for a licence to build a smelter in Saudi Arabia and Ma'aden maintains an interest in hearing about such projects," Ma'aden Chief Executive Abdullah Dabbagh said in response to Reuters' questions. "However, all such dialogue remains very general in nature and therefore there is no news to be given at this time." CHALLENGES Dubal's announcement comes months after mining giant Rio Tinto