TOKYO— Nintendo Co. is looking to expand tie-ups with software developers to strengthen its smartphone-game lineup after an early alliance aimed at cracking the fast-growing mobile market fell behind schedule, people familiar with the matter said.

Nintendo took a 10% stake in smartphone game maker DeNA Co. in 2015, and the two companies said they would introduce five smartphone games by March 2017. They have introduced only four so far, with the latest, “Animal Crossing: Pocket Camp,” coming in November.

By collaborating with other software developers in addition to DeNA, Nintendo believes that it could raise the pace of new titles, said people familiar with the plans. It isn’t planning to take ownership stakes in its new partners, they said.

Kyoto-based Nintendo long shunned smartphones, preferring to produce games for its own machines. More recently, company executives have said they see smartphones as a gateway to introduce casual gamers to Nintendo’s franchises and encourage them to trade up to its Switch console.

The global market for smartphone games rose 40% in 2016 to about $50 billion, according to Japanese data provider Famitsu, with Japan and China together accounting for more than 60% of the market and the U.S. about 20%.