Philippine President Rodrigo Duterte has stoked a feud with boxer Manny Pacquiao by criticising his "shallow" foreign policy knowledge, after the senator and eight-division champion said he found the leader's stand on the South China Sea was "lacking".

In a late night interview with SMNI news channel on Tuesday, Duterte said Pacquiao, a senator and close ally, should "study first" before weighing in.

Pacquiao stood his ground and hit back on Wednesday, when he said the country should pursue dialogue over disputes, but "stand strong in protecting our sovereign rights".

"I am a Filipino voicing out what needs to be said in defence of what has been adjudicated as rightfully ours," Pacquiao said, referring to a 2016 international arbitral ruling won by the Philippines in a case against China.