TOKYO (Reuters) - Japan's ambassador to China returned to Beijing on Monday after Tokyo summoned him over the weekend to discuss the escalating tension over a group of islands that both countries claim.

Foreign Minister Koichiro Gemba denied that the summoning of the ambassador, Uichiro Niwa, was meant as a protest to Beijing after Chinese patrol boats entered disputed waters near the islands, a ministry spokesman said.

The squabble over the islands, located near rich fishing grounds and potentially large oil and gas reserves, threatens to damage relations between Asia's two biggest economies.

Last week, the Chinese vessels twice entered waters near the islands, after Prime Minister Yoshihiko Noda said that Japan could buy the island from their private owners, instead of letting Tokyo governor Shintaro Ishihara go ahead with a similar plan.

Niwa, a former president of trading firm Itochu Corp, last month said he opposed Ishihara's plan, angering conservatives and drawing fire from the Foreign Ministry.

The two trading partners are trying to avoid a return to the acrimony that followed Japan's detention in 2010 of the skipper of a Chinese trawler whose boat collided with Japanese patrol ships near the islands -- called Senkaku in Japan, Diaoyu in China.

(Reporting by Mayumi Negishi, Editing by Jonathan Thatcher)