The Philippine foreign secretary says China has been notified of "red lines," or actions Manila would find unacceptable, in the South China Sea, including construction activities on a disputed shoal and extraction of oil and gas in disputed waters.

Foreign Secretary Alan Peter Cayetano provided on Monday more details of what President Rodrigo Duterte's administration has been doing to defend the country's sovereignty in the disputed region amid criticism that it has been far too soft on China.

Cayetano said: "At the right time, we will prove you wrong because nothing is secret forever."

He said the Department of Foreign Affairs has not fallen short in its diplomatic actions toward China.