President Halimah Yacob and Prime Minister Lee Hsien Loong have written to Bahrain's leaders to convey their condolences on the passing of Prince Khalifa bin Salman Al Khalifa, the world's longest-serving prime minister.

Prince Khalifa died at the age of 84 on Wednesday (Nov 11) while undergoing medical treatment in the United States.

President Halimah in her letter to King Hamad bin Isa Al Khalifa said she was saddened on hearing the news, and conveyed her deepest sympathies to the King and Bahrainis on behalf of Singaporeans.

"His Royal Highness was a committed statesman who dedicated his life to serving Bahrain. Over his five decades as Prime Minister, he played an integral role in modernising Bahrain and its economy," Madam Halimah wrote.

"He was a good friend of Singapore and received many of Singapore's leaders warmly during their visits to Bahrain.

"We were honoured to host His Royal Highness' visits to Singapore in 1976 and 2003. His Royal Highness shared the view that small states like Singapore and Bahrain should work closely together for mutual benefit," she added.

The letters from the Singapore leaders dated Friday, were released by Singapore's Ministry of Foreign Affairs.

PM Lee in his letter to Bahrain's new prime minister, Crown Prince Salman Bin Hamad Al Khalifa, extended his deepest condolences on behalf of the Singapore government.

The crown prince is also Deputy Supreme Commander of the Bahrain Defence Force.

"During His Royal Highness' long tenure as the first Prime Minister of Bahrain, he made significant contributions to the country's development and modernisation.

"Singapore appreciated his warm hospitality in hosting several Singapore leaders, including the late President S R Nathan, the late Minister Mentor Mr Lee Kuan Yew and Emeritus Senior Minister Mr Goh Chok Tong, during their visits to Manama," Mr Lee wrote.

"Our thoughts and prayers are with Your Royal Highness and the people of Bahrain during this time of mourning," he added.