"The United States welcomes ASEAN leaders’ insistence that South China Sea disputes be resolved in line with international law, including UNCLOS. China cannot be allowed to treat the SCS as its maritime empire. We will have more to say on this topic soon," the tweet read.

The UNCLOS refers to the 1982 United Nations Convention on the Law of the Sea.

The South China Sea is known in Vietnam as the East Sea.

Pompeo also included a link to the ASEAN Leaders' Vision Statement on a Cohesive And Responsive ASEAN: Rising Above Challenges And Sustaining Growth, which was passed by ASEAN leaders at the 36th ASEAN Summit in Hanoi on Friday.

Previously, on June 2, Pompeo had also tweeted a diplomatic note sent by U.S. Ambassador Kelly Craft to the U.N. in response to the one sent by the Permanent Mission of China to the U.N. on December 12. China was responding to Malaysia’s submission to the Commission on the Limits of the Continental Shelf.

"The U.S. rejects these maritime claims as inconsistent with international law as reflected in the 1982 Law of the Sea Convention," Craft’s note read.

Pompeo's Sunday tweet followed the Chairman's Statement of the 36th ASEAN Summit, in which ASEAN leaders expressed concern over recent South China Sea developments and called on all parties to build trust.

"We discussed the situation in the South China Sea, during which concerns were expressed over land reclamations, recent developments, activities and serious incidents, which have eroded trust and confidence, increased tensions and may undermine peace, security and stability in the region," the chairman's statement read.

It also called on member countries to strengthen mutual trust and refrain from activities that complicate or escalate disputes and harm peace and stability in the region. They also need to avoid actions that might complicate the situation and pursue peaceful resolutions and comply with international law, including the 1982 UNCLOS, the statement noted.

"We reaffirm that the 1982 UNCLOS is the basis for determining maritime entitlements, sovereign rights, jurisdiction and legitimate interests over maritime zones, as it sets out the legal framework within which all activities in the oceans and seas must be carried out."

The chairman's statement followed a series of provocative actions by China conducted in the South China Sea since the start of this year, while countries around the world were focused on battling the Covid-19 pandemic.

China formed the so-called "Xisha" and "Nansha" districts on Vietnam’s Hoang Sa (Paracel) and Truong Sa (Spratly) Islands, sank Vietnamese fishing vessels off the Paracels, unilaterally issued a fishing ban and sent a ship to tag Malaysia’s oil and gas exploration vessel.

China also recently announced a vegetable farming project on Woody (Phu Lam) Island of the Paracels to strengthen its illegal sovereignty claim. China also sent two diplomatic notes to the United Nations to make the infamous Four Sha claim, which covers a broader range than the notorious, illegal nine-dash line.

The 36th ASEAN Summit, with Vietnam as chair, took place online Friday because of the Covid-19 pandemic. Vietnam is also scheduled to chair the 37th ASEAN Summit at the end of this year.