SINGAPORE - President Halimah Yacob and Prime Minister Lee Hsien Loong have written to Indonesia's President Joko Widodo to express their condolences on the sinking of the KRI Nanggala-402 submarine.

Preparations are being made to salvage the submarine, which had split into at least three parts, and retrieve the bodies.

President Halimah on Monday (April 26) wrote: "I was saddened by the loss of lives of the 53 crewmen that resulted from the sinking of the KRI Nanggala-402 submarine.

"We were praying and hoping that they could be saved but it turned out otherwise.

"This is a trying period for the people of Indonesia and the families and loved ones of the victims, who were performing their duty to their nation when the accident occurred.

"On behalf of the people of Singapore, I extend my deepest condolences to the people of Indonesia.

"My prayers and thoughts are with the families and loved ones of the crewmen on their terrible loss and may they find peace in this month of Ramadan."

PM Lee in his letter to President Widodo said: "I was deeply saddened by the sinking of the KRI Nanggala-402 submarine and the tragic loss of lives.

"My heartfelt condolences to the families and loved ones of the crew members affected by this devastating incident.

"Singapore's MV Swift Rescue was part of the extensive search and rescue operation led by your government. Singapore stands ready to provide any further assistance that may be required.

"My sympathies and thoughts are with the people of Indonesia in this time of grief."