Foreign Affairs Secretary Albert del Rosario said China's efforts were aimed at undermining a United Nations tribunal that is due to rule early next year on a Philippine challenge to its claims to the disputed waters.
"China is accelerating its expansionist agenda and changing the status quo to actualise its nine-dash line claim and to control nearly the entire South China Sea before... The handing down of a decision of the arbitral tribunal on the Philippine submission," del Rosario told reporters.
But the nine dashes are in some places more than 1,000 (600 miles) from the nearest major Chinese landmass and well within the exclusive economic zones of its neighbours.
The dispute - with Vietnam, Malaysia, Brunei and Taiwan also claiming parts of the sea - has for decades been a source of deep regional tension and occasional military conflict.
Also Read
Tensions have escalated sharply in recent years as China has moved to increase its presence and assert its authority in the waters.
"China also made and continues to make incursions in the West Philippine Sea and undertake massive reclamation activities in the disputed areas," he said, referring to the Philippine-claimed waters by its local name.
Del Rosario said the reclamation works were taking place on all seven reefs that China occupies in the Spratly Islands, one of the biggest archipelagos in the sea between the Philippines, southern Vietnam and Malaysia.
China is a signatory to the UN's Convention on the Law of the Sea, a treaty that is meant to govern nations' maritime actions.