China claims almost the entire South China Sea, believed to have huge deposits of oil and gas, and is building islands on reefs to bolster its claims. Brunei, Malaysia, the Philippines, Taiwan and Vietnam also have claims to parts of the waters, through which about $5 trillion in trade is shipped every year.
China also has a separate dispute with Japan over a group of uninhabited islets in the East China Sea.
Chinese Foreign Ministry spokesman Lu Kang told a daily news briefing that it had thought the G7 foreign ministers' meeting in the Japanese city of Hiroshima wouldn't really have anything to do with China.
Once the statement came out, China decided there were certain "incorrect and mistaken" parts in it, Lu said, so China had to make its position clear.
"So yes we called in envoys from the relevant countries, solemnly expressing China's position on this matter," he added, as reported by Reuters.
China informed the envoys of exactly what it had said in public on the issue, Lu said.
The Foreign Ministry on Tuesday expressed anger at the G7 statement, saying the grouping should not taking sides on issues involving territorial disputes.
The G7 is composed of the United States, France, Canada, Germany, Great Britain, Italy and Japan.
G7 leaders will meet again in Japan for their summit next month.