Japan and Southeast Asian countries called for freedom of the air and sea on Saturday, as China's military assertiveness raises regional tensions and after US and Chinese warships narrowly avoided collision in the disputed South China Sea.

China's recent announcement of an air defense zone covering disputed islands in the East China Sea and its territorial claims in the South China Sea have raised concerns that a minor incident in the disputed seas could quickly escalate.

The near-miss between a US guided missile cruiser, USS Cowpens, and a Chinese warship operating near China's only aircraft carrier, the Liaoning, last week was the most significant US-China maritime incident in the South China Sea since 2009, said security expert Carl Thayer at the Australian Defense Force Academy.

"It is a gravely disturbing development," said Ian Storey, a regional security analyst at Singapore's Institute of South East Asian Studies.

"If China continues to challenge the presence of foreign naval ships in the South China Sea, it is only a question of time before a serious and potentially deadly incident occurs."

Japanese Prime Minister Shinzo Abe and leaders of the Association of Southeast Asian Nations (Asean) agreed at a summit in Tokyo on the need for freedom of the high seas and skies and the peaceful resolution of disputes.

The statement did not criticize China's new air zone, which has triggered protests from Japan, United States and South Korea. Many Asean members have deep economic ties with China.

But Abe himself minced no words at a later news conference.

"The air defense identification zone China has established in the East China Sea is unjustly violating the freedom of aviation over the high seas, which is a general rule in international law. We are demanding China rescind all measures like this that unjustly violate the general rule," Abe said.

Sino-Japanese tensions have risen over the past year in a long-running dispute over Japanese-controlled islands in the East China Sea that are also claimed by Beijing. Both countries and have scrambled aircraft and conducted naval patrols in the area.

China and several Asean nations have competing territorial claims in the energy-rich South China Sea.

The Japan-Asean summit is the centerpiece of a three-day regional gathering officially billed as celebrating 40 years of diplomatic ties.

"I would like to build an Asia-Pacific future that respects each other's cultures and construct an economic system that is realized not by force, but by rule of law and our efforts," Abe said at the summit.

The US Pacific Fleet said in a statement on Friday that the USS Cowpens, operating in international waters in the South China Sea, last week narrowly missed colliding with the Chinese warship.

The Liaoning aircraft carrier, which has yet to be fully armed and is still being used as a training platform, was flanked by escort ships including two destroyers and two frigates during its first deployment into the South China Sea.

One of the escort ships maneuvered near the Cowpens in the incident on December 5 and the Cowpens took evasive action.

"Eventually, effective bridge-to-bridge communications occurred between the US and Chinese crews, and both vessels maneuvered to ensure safe passage," said the defense official.

The United States had raised the incident at a "high level" with China, according to a State Department official quoted by the US military's Stars and Stripes newspaper.

Beijing has yet to comment, but China's often-nationalistic on-line platforms were filling with debate about the near-miss. One poster demanded the Chinese navy follow up by blazing an "independent sea lane" to Hawaii.

Asked if the Chinese vessel was moving towards the Cowpens with aggressive intent, a US defense official declined to speculate.

"Whether it is a tactical at-sea encounter, or strategic dialogue, sustained and reliable communication mitigates the risk of mishaps, which is in the interest of both the US and China," the official said in an email to Reuters.

Beijing routinely objects to US military surveillance operations within its exclusive economic zone, while Washington insists the United States and other nations have the right to conduct routine operations in international waters.

The US Navy said the Cowpens was conducting regular freedom-of-navigation operations when the incident occurred.

China deployed the Liaoning to the South China Sea just days after announcing its air defense zone which covers air space around a group of tiny islands in the East China Sea that are administered by Japan but claimed by Beijing as well.

Japanese lawmakers called China's actions "reckless and risky measures", prompting Beijing to accuse Tokyo of being a regional aggressor.

Chinese state media kept up the invective against Japan's complaints over the air space zone on Friday, with the official Xinhua news agency saying Abe was going to "stage again its China-is-to-blame game" at the Asean summit.

"It is believed that anyone with only half a brain knows that it is Japan who intentionally set the region on fire in the first place," Xinhua said in an English-language commentary.-Reuters