A merger of Russian cellphone operator Vimpelcom and Ukrainian mobile leader Kyivstar was ranked as the world's biggest deal this week, according to Thomson Reuters data released on Friday.

Russia's Alfa and Norway's Telenor, the main Vimpelcom shareholders, announced on Monday their plans to merge Vimpelcom and Kyivstar and create a leading emerging markets' mobile operator.

A $3.7 billion bid by Spain's Telefonica for Brazilian telecommunications company GVT came second, followed by a $3 billion Cisco Systems Inc deal to buy Norwegian videoconferencing company Tandberg.

Despite the Vimpelcom deal, worth an estimated $11.7 billion and ranked as the largest acquisition by a Russian company this year, total acquisitions by Russian firms were down 53 percent at $28.6 billion in the year-to-date, the data showed.

The decline was deeper than a 38 percent drop in the volume of overall worldwide mergers and acquisitions, which has come to $1.5 trillion since the beginning of 2009.

The volume of M&As had been growing rapidly in Russia before the global financial crisis hit last year, closing international debt markets for potential buyers which also saw debt servicing costs soar on the back of the rouble depreciation.

But as capital markets have started to recover, the M&A business may pick up again, with financially stronger players already turning their eyes on indebted peers.

X5 Retail Group, Russia's largest grocer by sales, last week applied for regulatory approval to buy Paterson -- a grocery chain in debt restructuring process.

(Writing by Maria Kiselyova; Editing by David Cowell)