East Asian leaders on Tuesday condemned North Korea's development of nuclear weapons and ballistic missiles and urged it to comply with U.N. Security Council resolutions banning it from conducting nuclear and missile activities, according to a senior Japanese official.

Also, in a veiled criticism to Beijing's assertive claims in the South China Sea, the leaders underscored the importance of peacefully settling disputes in the waterway in accordance with international law, the official told reporters after the 18-nation East Asia Summit in Manila.

U.S. President Donald Trump joined other leaders over lunch but skipped the plenary session due to a nearly two-hour delay and left for home around the time it started. He told reporters aboard Air Force One that he delivered his prepared remarks during the lunch instead.

Secretary of State Rex Tillerson attended in his place.

During the summit, Japanese Prime Minister Shinzo Abe urged the international community to increase pressure on North Korea to "the maximum level" through measures such as reviewing diplomatic ties, tighter export and import control, as well as more restrictive measures on North Korean laborers, according to the official.

Abe urged China and Russia -- two countries that have close ties with North Korea -- to do more to rein it in.

"We need to push (North Korea) into a situation that would force it to tell us it wants to hold talks on giving up its nuclear and missile development," he was quoted by the official as saying.

Japan and other countries "can never expect to have meaningful dialogue with North Korea unless it shows willingness to have talks toward its denuclearization," he added.

Similarly, Thai Prime Minister Prayut Chan-o-cha told his summit partners that North Korea must rid itself of nuclear weapons, abide by U.N. Security Council resolutions and return to talks for denuclearization, a Thai government spokesman said.

The meeting came as North Korea has been stepping up its development of nuclear-tipped missiles capable of reaching the continental United States. In the face of such a threat, the Trump administration says it is keeping all options -- including military action -- on the table in dealing with the defiant regime.

China opposes North Korea's nuclear weapons development program but fears strong economic pressure could trigger a regime collapse, resulting in the loss of a strategic buffer zone between itself and South Korea where the U.S. maintains a large military presence.

China accounts for about 90 percent of North Korea's total trade and is a major supplier of oil to the country, leading critics to call Beijing an economic enabler of Pyongyang's nuclear weapons and ballistic missile programs.

Separately, Abe called for increased international pressure on North Korea to achieve an early resolution to Pyongyang's abduction of Japanese nationals in the 1970s and 1980s.

Abe and several other leaders, meanwhile, expressed concern about China's unilateral construction and militarization of outposts in disputed areas of the South China Sea, according to the senior Japanese official.

They stressed the importance of ensuring non-militarization and self-restraint in defusing tensions over the issue, the official said.

Refuting such a view, Chinese Premier Li Keqiang told the summit that Beijing will "firmly safeguard" the freedom of navigation and overflight in the South China Sea, the official Xinhua News Agency said.

"China shows more interest in safeguarding peace, stability, and navigation freedom in the South China Sea than any other country in the world," Li was quoted as saying.

Abe urged China and the Association of Southeast Asian Nations to craft an "effective" code of conduct" at an early date in an effort to defuse tensions in the South China Sea.

On Monday, Li and the ASEAN leaders agreed to start consultations on the text of the code. An ASEAN diplomatic source said the two sides plan to start such talks in March in Vietnam.

China has overlapping territorial claims with Brunei, Malaysia, the Philippines, Vietnam and Taiwan in the South China Sea, a strategic waterway through which over one-third of global trade passes.

China's attempts to force a shift in the status quo in the disputed waters have drawn international condemnation. Beijing has also refused to comply with last year's international tribunal ruling that invalidated the country's claims across almost the entire sea.

The East Asia Summit comprises ASEAN -- which groups Brunei, Cambodia, Indonesia, Laos, Malaysia, Myanmar, the Philippines, Singapore, Thailand and Vietnam -- plus Australia, China, India, Japan, New Zealand, Russia, South Korea and the United States.