The Philippine government says it has conveyed “serious concerns” to the Chinese embassy over reports that Chinese ships unloaded building materials and put up military posts on reefs claimed by Manila in the South China Sea.

The foreign ministry said Wednesday the Philippine military observed the activity near what is known as the Iroquois or Amy Douglas Bank, about 230 kilometers west of the Philippine island of Palawan. It said the site is well within the Philippines' 370-kilometer exclusive economic zone.

The ministry said a marine surveillance vessel and other Chinese naval ships unloaded the materials, erected an unknown number of posts and placed a buoy near the Iroquois Bank on Tuesday. It said the action was a clear violation of a 2002 agreement between China and Southeast Asian nations on conduct in the South China Sea.

It is the second diplomatic flare-up within a week concerning overlapping claims to reefs and islands in the South China Sea. Vietnam complained late last week that Chinese naval vessels had cut a cable trailing from an oil exploration ship and forced the ship out of waters off Vietnam's southeastern coast.

Vietnam says that incident occurred about 600 kilometers south of China's Hainan Island and 120 kilometers off its coast, well within Vietnam's exclusive economic zone.

At a Chinese foreign ministry briefing Tuesday, spokeswoman Jiang Yu said the Vietnamese survey ship had been operating illegally in Chinese-administered waters and that the action against it was “completely justified.”

There was no immediate response from China to the Vietnamese complaint. However the Philippines said both countries remained committed to maintaining peace and stability in the area.

The Iroquois Bank lies near the Spratly Islands, which are claimed in whole or part by China, the Philippines, Vietnam, Malaysia, Brunei and Taiwan.