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.
China insists it has sovereign rights to nearly all of the resource-rich sea, even areas approaching the coasts of the Philippines and other Southeast Asian nations, based on an old Chinese map with nine dashes outlining its territory. 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.
Tensions have escalated sharply in recent years as China has moved to increase its presence and assert its authority in the waters. Del Rosario said those activities were continuing to pick up pace, pointing to what he described as Chinese ships ramming Filipino fishing boats at a shoal close to the Philippine coast in January.
"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.
"The alterations of these features are plainly intended to change the character, status and maritime entitlements of the said features, which prejudice the arbitration and undermine the work of the arbitral tribunal to hear and objectively decide the case," he said in a speech to the foreign journalists' association in Manila.
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.