Clinton was expected to discuss the North Korean issue further during talks with ministers at the forum.
She was due to meet Yang Jiechi, her Chinese counterpart, on Thursday to urge Beijing to put more pressure on the North over its nuclear programme.
Nuclear weapons ambitions
Clinton had announced the new measures against Pyongyang during a visit to South Korea on Wednesday.
She said the sanctions were designed to stamp out illegal money-making ventures used to fund North Korea's nuclear weapons programme.
The sanctions would be aimed at the sale or procurement of arms and related goods as well as the procurement of luxury items.
Clinton also used her visit to urge North Korea to accept responsibility for the sinking of a South Korean warship in March, which left 46 sailors dead.
The US has announced a series of joint military exercises aimed at deterring the North from any future attack.
Robert Gates, the US defence secretary, and his South Korean counterpart Kim Tae-Young said the drills off the Korean peninsula's western coast were designed "to send a clear message to North Korea that its aggressive behaviour must stop".
However, Ri on Thursday criticised the planned naval drills, saying the military exercises pose a major threat to global peace.
"The decision to hold military drills is a major danger for the security of the region," he said.
"Such movements pose a great threat not only to the peace and security of the Korean peninsula but also to global peace and security."
China has also expressed concern about the exercises.
The US has sent the 97,000-tonne aircraft carrier USS George Washington to take part in the drill set to begin on Sunday in the Sea of Japan.
The exercises will involve about 20 ships and 200 fixed-wing aircraft, according to military officials.
The drills are the first overt military response to the sinking of the Cheonan, and underscore US support for Seoul.
The North has denied it is responsible for the incident and a United Nations Security Council statement condemning the sinking did not name North Korea as the culprit, apparently after Chinese pressure.
The diplomatic row surronding the sinking, and upcoming military exercises, is playing out at the meeting of the Association of Southeast Asian Nations (ASEAN) in Hanoi, the Vietnamese capital.
The US, China and both Koreas are attending meetings of the regional body and are trying to influence the final communique which will be issued on Friday.
North Korea and its main ally, China, are pushing to avoid tough language, while the US and South Korea favor harsh words for North Korea.
Earlier in the week, the ASEAN foreign ministers said they "deplored" the ship sinking, but characterised it as an "incident" instead of an "attack", to the dismay of South Korea.