Asean foreign ministers urged self-restraint and non-militarisation in the South China Sea in a joint communique that came at the end of their annual meeting, with several expressing concerns about the "land reclamations, activities, and serious incidents in the area".
Such activities eroded trust and confidence and increased tensions, they said.
The ministers also highlighted the need to pursue peaceful resolution of disputes in accordance with international law, including the 1982 United Nations Convention on the Law of the Sea - with the United States later giving the assurance that it stood alongside Asean in standing up for the rule of law.
The communique, which did not refer to either China or the US, was released a day after the 53rd Asean Foreign Ministers' Meeting ended, one of several annual regional gatherings last week that took place virtually owing to the Covid-19 pandemic.
China's territorial claims, which span almost the entirety of the South China Sea and clash with those of Asean countries Vietnam, the Philippines, Brunei and Malaysia, were rejected as illegal by an international arbitral tribunal in 2016, a ruling which the US explicitly sided with for the first time in July this year.
On Friday, US Secretary of State Mike Pompeo emphasised the "enduring US commitment to Asean", in championing the rule of law, respect for sovereignty, as well as transparency in the South China Sea and Mekong region.
"We stand with our Asean partners as we insist on the rule of law and respect for sovereignty in the South China Sea, where Beijing has pursued aggressive campaigns of coercion and environmental devastation," he said in a statement.
Tensions between the US and China have escalated sharply in recent months, over a wide range of issues including espionage, Chinese social media apps, a national security law for Hong Kong and China's military build-up in the South China Sea.
Last month, the US announced sanctions on 24 Chinese state-owned enterprises linked to the building and militarising of outposts on artificial islands in the South China Sea. China responded by firing a series of missiles into the South China Sea.
During the US-Asean Foreign Ministers' Meeting on Thursday, Mr Pompeo urged Asean to reconsider business dealings with Chinese enterprises "that bully Asean coastal states in the South China Sea".
He also said Washington is deeply concerned about 12 Hong Kong democracy activists who have been detained in China, noting that they have been denied access to lawyers.
"We also reiterate that the Chinese Communist Party's imposition of draconian national security legislation has obliterated Hong Kong's autonomy," he said, referring to the Chinese law which targets acts deemed to be secessionist, subversive, terrorism or foreign collusion.
He also said the US plans to co-host with Vietnam the third Indo-Pacific Business Forum, which aims to foster millions of dollars in US investment. Washington will also provide US$5 million (S$6.8 million) for skills training under a Young South-east Asian Leaders Initiative Academy in Vietnam.
Regional watchers said the appeal to boycott certain Chinese firms was unlikely to be well received by Asean countries, which are generally wary of being drawn into great-power competition.
China's Foreign Minister Wang Yi on Wednesday told the Asean ministers that the US was interfering in the South China Sea and was driving its militarisation.
Meanwhile, Mr Pompeo also singled out North Korea and Myanmar as challenges facing the region. He urged North Korea to end provocations and engage in sustained negotiations to achieve complete denuclearisation, and expressed concerns over the continued violence in Myanmar's Rakhine state.