BEIJING -- A member of Japan's coalition government arrived in Beijing on Tuesday with a letter for the head of the Communist Party, Xi Jinping, from the Japanese prime minister, Shinzo Abe, to try to help calm an escalating dispute between the two countries over contested islands in the East China Sea, Japanese officials said.
Separately, the Philippines announced on Tuesday that it would formally challenge China's claims in the South China Sea before a United Nations tribunal that oversees the Convention on the Law of the Sea.
The Philippines has been in a bitter argument with China since last spring, when China effectively took control of a series of rocky outcroppings in the South China Sea known in the Philippines as Scarborough Shoal and in China as Huangyan Island.
The Philippine secretary of foreign affairs, Albert del Rosario, said in Manila that China's claim to much of the South China Sea was "unlawful." China has "interfered with the lawful exercise by the Philippines of its rights within its legitimate maritime zones," Mr. del Rosario said.
He emphasized that resorting to the tribunal meant that the Philippines could "present our case against China and defend our national interest and maritime domain before an independent international tribunal." International law, he said, will be "the great equalizer."
China drew up a map in the late 1940s that marked its territorial claims in the South China Sea -- by some estimates, about 80 percent of the sea -- with what it refers to as the nine-dash line. The country has consistently said it will not agree to arbitration on counterclaims by an international tribunal. Legal experts said that a matter brought before such a panel required negotiations, and that without China's presence it was unlikely that a proceeding could take place.
"This is what I don't see taking place," said Jay L. Batongbacal, an assistant professor of law at the University of the Philippines in Manila.
A Chinese expert on the Asia-Pacific region, Su Hao of China Foreign Affairs University in Beijing, agreed with that assessment.
Both China and the Philippines need to agree on arbitration for the case to proceed, Professor Su said. "The Philippines action is ineffective," he said. "It's making trouble out of nothing."
Aside from China and the Philippines, three other countries in Southeast Asia -- Brunei, Malaysia and Vietnam -- make claims to islands in the South China Sea. So does Taiwan.
China's increasingly aggressive claims in the South China Sea, and the tensions with Japan in the East China Sea, have raised concerns in the Obama administration as Washington has indicated that it plans to strengthen its military presence in the Asia-Pacific region.
The Philippines, an American ally, has felt particularly aggrieved because China has kept patrol boats in the waters around Scarborough Shoal, preventing Filipino fishermen from using their traditional fishing grounds in a lagoon there.
Washington brokered an agreement last spring that called for the Philippines and China to withdraw government vessels from the area, American officials said. But after complying, China sent surveillance ships back, and stretched a cable across the mouth of the lagoon, preventing Filipino fishermen from venturing there, the officials said.
The Philippines plans to contest all of China's claims in the South China Sea, not just its claims on Scarborough Shoal, Mr. del Rosario said.
In the dispute between Japan and China, it was not immediately clear whether the visiting Japanese politician -- Natsuo Yamaguchi, the leader of the New Komeito Party, which is considered less hawkish than the governing Liberal Democratic Party -- would meet with top Chinese officials.
An official of the China-Japan Friendship Association, which appeared to be handling Mr. Yamaguchi's visit, said after his arrival that the schedule for Mr. Yamaguchi during his stay in Beijing had not been made final.
Mr. Yamaguchi's visit comes amid a drumbeat of bellicose commentary in the Chinese state-run news media about the need for China's military to prepare for war, and criticism of Mr. Abe for trying to form alliances with China's neighbors in Southeast Asia.
The feud over the islands, known as the Diaoyu in China and the Senkaku in Japan, reached a dangerous new level nearly two weeks ago, when both Japan and China scrambled jet fighters over the East China Sea. The United States is obligated under a security treaty with Japan to defend the islands, which were handed back to Japan by Washington in 1972 as part of the return of Okinawa.
In a speech in Hong Kong on Wednesday, a former Chinese diplomat, Ruan Zongze, said China wanted a peaceful resolution of border issues.
"We are absolutely committed to peaceful resolution, peaceful dialogue," said Dr. Ruan, a vice president at the China Institute of International Studies, a research group in Beijing that is affiliated with the Foreign Ministry.
Dr. Ruan said the Chinese military remained under the control of the Communist Party. "Even if the military wants to be more aggressive, the party will push the brake," he said in an interview before his speech at the Foreign Correspondents' Club in Hong Kong.
Keith Bradsher contributed reporting from Hong Kong, and Bree Feng contributed research from Beijing.
Correction: January 22, 2013, Tuesday
This article has been revised to reflect the following correction: An earlier version of this article misstated the timing of the Philippines' argument with China over its claims in the South China Sea, and the timing of a temporary resolution brokered by the United States. The standoff and the resolution both took place last spring, not last summer.
This article originally appeared in The New York Times.