MANILA (REUTERS) - Philippine leader Rodrigo Duterte suffered the biggest ratings slump of his presidency in the third quarter, an independent pollster said on Tuesday (Sept 25), amid signs of public unease about rising inflation and the cost of staple foodgrain rice.

Duterte's approval rating, a measure of his performance, dropped 13 points to 75 per cent in a survey of 1,800 voting-age Filipinos conducted by Pulse Asia this month.

His trust rating, which gauges sentiment about his personality, fell 15 points to 72 per cent. A similar poll in June had scored him at 88 per cent and 87 per cent respectively.

Presidential spokesman Harry Roque said Duterte, who has been in office since June 2016, was unperturbed by ratings and was focused on doing his job.

"We are not affected by it because the president will do his best to discharge his duties," Roque told a regular news briefing. "I believe that's still a very good approval rating."

Roque said rising inflation could have caused the drop in the ratings because higher prices for rice, have negated the administration's gains in other areas.

The survey was conducted in the first week of September, when official data showed annual inflation at 6.4 per cent in August, the highest in nearly a decade.

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It came amid calls to abolish the grains import agency and in a week when a graft complaint was made against its chief, who has since resigned.

Another pollster, Social Weather Stations, released results this month of a second-quarter poll that showed trust in Duterte had dropped to its lowest level during his presidency, although he maintained a rating of "very good".