Singapore on Saturday apologized for the two-hour detention of two senior Indonesian officials at Changi Airport on Thursday, but said the questioning was part of routine random checks for all visitors to the city-state.

Immigration officers apologized to presidential advisor Adnan Buyung Nasution and former attorney general Abdul Rachman Saleh after the interrogation, the Singapore ministry of foreign affairs said.

"The questioning was made to ensure thorough checking of visitors. After the questioning, (Buyung) was allowed to enter Singapore freely," the ministry said as quoted by detik.com.

Buyung, one of President Susilo Bambang Yudhoyono's advisers, and Abdul Rachman reportedly flew to Singapore for medical purposes.

They arrived at Changi Airport on Thursday night and were immediately taken by military personnel to an immigration room for two hours of questioning, said Buyung's assistant Bunga Surawijaya.

Bunga said the two men were relaxed at first but later turned impatient when the questioning lasted for over an hour with no sign of ending.

They were released after Buyung contacted former Indonesian foreign minister Ali Alatas for help, Bunga said.

"We hope that visitors to Singapore can understand our security measures in this extraordinary event," the ministry said. "The current situation prompts us to (make random checks), considering challenges involved in keeping security," it added.

"We never mean to harm anyone's feelings. This is for the safety of everyone, including Singapore's visitors. We hope you can understand."

The brief detention of the two officials came amid heightened security at Changi Airport following the recent escape of terrorist Mas Slamet Kastari from a jail in the city-state.

Both Buyung and Abdul reportedly arrived back in Indonesia at 6 p.m. at Soekarno-Hatta Airport.

Indonesia Foreign Ministry spokesman Kristiarto Legowo said his office would ask for an explanation Monday from Singapore over the lengthy questioning. (anw)