Former US president Bill Clinton and his wife, former Secretary of State Hillary Clinton, paid tribute to the late Mr Lee Kuan Yew, saying he transformed Singapore into one of the world's strongest and most sustainable economies.

"Hillary and I mourn the passing of Lee Kuan Yew,'' the Clintons said in a statement.

"Lee transformed Singapore into one of the world's strongest and most sustainable economies. He also firmly established his country as an important friend and partner of the United States.

"After leaving office, he continued to offer brilliant analysis and wise advice to those who sought it. We will always be grateful for our fascinating conversations with him over the years,'' they said.

"Our thoughts are with Prime Minister Lee Hsien Loong, his entire family, and all the people of Singapore."

Mr Lee died on Monday morning aged 91.