BANGKOK (Reuters) - A U.S. envoy denounced Chinese “intimidation” in the South China Sea at a summit of Southeast Asian leaders on Monday and said they should not be bullied into giving up their resources by what he compared to a conquest.

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The raised rhetoric from White House National Security Adviser Robert O’Brien at the Association of Southeast Asian Nations (ASEAN) meeting in Bangkok drew a rebuke from China, which lays claim to most of the busy waterway.

“Beijing has used intimidation to try to stop ASEAN nations from exploiting the off-shore resources, blocking access to 2.5 trillion dollars of oil and gas reserves alone,” O’Brien told the ASEAN-U.S. summit in a speech.

Disputes should be handled peacefully, he said later.

“We don’t think they should be handled by intimidation or through maritime militias or by random ships or by surrounding islands ... That’s just not how things should be done in the 21st Century. That’s conquest.”

Chinese Vice Foreign Minister Le Yucheng, also in Bangkok for the meetings, said it was unacceptable for countries from outside the region to come “to make waves, escalate disputes and create tensions”.

He emphasized progress on the code of conduct for the South China Sea which China is discussing with ASEAN countries.

O’Brien delivered an invitation from Trump to the ASEAN leaders to a special summit at some point in the first quarter of 2020.

At this year’s summit, U.S. Commerce Secretary Wilbur Ross was the highest-ranking delegation official, prompting the 10-member ASEAN to downgrade its participation at the meeting to only leaders from Thailand, Vietnam and Laos.

China was represented at meetings in Bangkok by its premier, Le Keqiang.

Despite the South China Sea accusations, O’Brien said Washington sought a “great relationship” with China and that the two sides were close to a “phase one” agreement to begin to roll back a 16-month trade war.