Didi Chuxing, the biggest ride-sharing firm globally by the number of trips, was "immensely saddened by the tragedy," it said in a statement.

"We are deeply sorry. We fell short of your expectations. We can't and we won't shirk from our responsibilities," it said.

The incident took place as Didi has been stepping up safety measures including for its carpooling service, Didi Hitch, after the murder of a flight attendant in May sparked wider community outrage.

The steps include limiting Didi Hitch drivers to picking up passengers of the same sex during early morning and late evening hours.

Police in Wenzhou's Yueqing city said on their official microblog that the 20-year-old woman, surnamed Zhao, got into a Didi carpool vehicle at 1 p.m. on Friday, and sent a message to a friend at around 2 p.m. seeking help before losing contact.

Police said they found the suspect, a 27-year-old driver from Sichuan province surnamed Zhong, at about 4 a.m. on Saturday. They said Zhong confessed, and that the victim's body had been recovered and an investigation was continuing.

Didi said the driver had passed background checks and had logged in with his authentic ID and had been confirmed by facial recognition on the day. However, the driver had altered the car's license plate before the trip, the company said.

A day earlier, a passenger had complained to Didi that the same driver had repeatedly requested that she sit in the front seat, drove to a remote area, and followed her "for a distance" after she got out of the car, Didi said. It said the customer service representative who took the complaint had not followed up with an investigation within two hours, as the firm promises.

Didi Chuxing - which has been valued at $50 billion and counts SoftBank Group Corp as a major investor - is aggressively expanding overseas, targeting new markets in Mexico, Brazil and Australia, going head-to-head with Uber. In 2016, Didi acquired Uber's China business.

The latest case was also trending on the Twitter-like Weibo platform, with seven related topics in the top 50 on Saturday afternoon.

REUTERS