A merger between Chinese ride-hailing firm Didi Chuxing and the China unit of U.S. rival Uber could be facing its first hiccup. That challenge comes from China's anti-trust regulator.

"The Ministry of Commerce hasn't received any files from Didi and Uber yet. According to the regulation and anti-trust law, companies need to declare such deals to the Ministry of Commerce. Those who don't declare could not conduct such deals," said Shen Danyang, Commerce Ministry spokesman.

The Ministry of Commerce is one of China's anti-trust regulators and it says that the two companies need to seek approval for the deal to go ahead. 

It had been unclear previously whether such a filing would be required as both firms are loss-makers in China. Didi and Uber have been in a fierce battle in China, spending billions of dollars to subsidize rides and gain users. Other players, however, could step up the competition.