WASHINGTON - The US congratulated Thailand on its general election on Sunday in which the opposition party declared victory, the State Department said on Sunday.

"We congratulate the people of the Kingdom of Thailand, our long-time friend and ally, for their robust participation in the July 3rd parliamentary elections," State Department spokesperson Victoria Nuland said in a statement.

She said the US looked forward to working with the next Thai government to "broaden and strengthen" their alliance.

"The United States also looks forward to working with the opposition and non-partisan civil society, as we have always done, " she added.

Thailand's opposition Phue Thai Party's prime minister candidate Yingluck Shinnawatra declared victory while ruling Democrat Party leader Abhisit Vejjajiva conceded his party's defeat hours after the voting ended in the Southeast Asian nation' s general election on Sunday.

Yingluck Shinawatra, the youngest sister of self-exiled former Prime Minister Thaksin Shinawatra, is set to become the country's 28th and the first female prime minister.