Visitors to three popular beauty spots have recently complained about a spate of littering - and they believe they know who the culprits are.
The trash, usually found in spots near military training grounds, is suspected by hikers to have been left by those who were in the vicinity for military exercises.
Empty food ration packets, mess tins and transparencies bearing map markings have been found around Upper Peirce and Seletar reservoirs, as well as a beach near Pasir Ris.
Ms Debbie Fordyce, a rights activist, has been walking at the reservoirs for more than a decade and picks up litter when she sees it.
On April 10, she collected 64 empty ration packets, 23 clear plastic bags used to hold these rations and other items such as water bottles wrapped in green socks. "The trash filled seven full plastic bags," she said. Two weeks later, when she visited the same clearing, she and her nephew - together with this reporter - collected four bags of litter.
A biker, who wanted to be known only as Mr Koh, said he sees the trash on his route towards Seletar Reservoir every Sunday. The 43-year-old bank manager said: "The trash is usually scattered... and looks to be the work of irresponsible individuals, not an organisation.
"The sight is an eyesore, because the forest is a reserve meant for people who want to get away from the city. We don't need reminders of inconsiderate and uncivil behaviour."
As training grounds are out of bounds to civilians, the trash that the public found may be part of a larger litter problem.
A clean-up operation by the Public Hygiene Council on April 23 at a beach near Pasir Ris yielded at least 100 empty food ration packets.
Council chairman Edward D'Silva said: "Unfortunately, military trash found in public spaces is not something new."
Wildlife consultant Subaraj Rajathurai urged the public not to quickly point fingers at the Ministry of Defence as the items could also be left behind by the public. He noted that trekkers may "want a realistic experience, and they buy military rations from the army market".
The empty ration packets on the beach, he added, could have been washed ashore by contractors who did not dispose of the trash properly.
"More investigation is needed," he said.
Colonel Andrew Lim, assistant chief of the general staff (training), said the Singapore Armed Forces (SAF) "does not condone littering, whether in camps or in the training area". Servicemen are educated and consistently briefed on the need to keep the training areas clean and to dispose of waste appropriately, he said, noting that administrative areas are cleared of waste before units depart the training area.
Col Lim added: "We do our best to keep them litter-free. However, from time to time, we do receive public feedback of SAF litter in training areas.
"When that happens, the SAF will conduct clean-up operations. At the same time, servicemen caught littering will be disciplined accordingly."
Member of Parliament Lee Bee Wah, who is also chairman of the Government Parliamentary Committee for the Environment and Water Resources, said there is "no magical answer" to the littering problem.
Ms Lee, a vocal advocate against littering, said: "We just need to go on emphasising to the public the need to be responsible citizens.
"Hopefully, in time, we win (litterbugs) over... We have set the ball rolling and we just have to keep at it."