China is claiming sovereignty over a series of reefs also claimed by the Philippines in the latest in a series of territorial disputes in the South China Sea.

A foreign ministry spokesman said in Beijing Thursday it is “entirely normal” for a Chinese enterprise to have unloaded building materials and erected marker posts in the area. Spokesman Hong Lei said the reefs are in Chinese territorial waters.

Philippine officials said Wednesday they had conveyed “serious concerns” to the China about the activity, which was first noticed by Philippine fishermen and reported to the Philippine navy.

The Philippines said the activity took place 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 Philippines complained previously to Beijing, charging that Chinese patrol boats entered Philippine waters near the disputed Spratly islands in March and ordered an oil exploration boat out of the area.

Late last week, Vietnam complained 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. China said the survey ship had been operating illegally in Chinese-administered waters.

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

The flare-ups come ahead of a major security forum in Singapore that will be attended by defense ministers and military chiefs from around the Asia-Pacific region.