Singapore urged Indonesia to take "urgent measures" on Monday as severe air pollution from a rash of forest fires on Sumatra island choked the densely populated city-state.
Singapore's skyscrapers including the famous Marina Bay Sands casino towers were shrouded in haze and the acrid smell of burnt wood pervaded the central business district.
Parts of neighbouring Malaysia were also suffering from the smoky haze, a recurring problem Southeast Asian governments have failed to solve despite repeated calls for action.
Singapore's National Environment Agency (NEA) said it had alerted its Indonesian counterpart on the situation "and urged the Indonesian authorities to look into urgent measures to mitigate the transboundary haze occurrence".
But the Indonesian forestry ministry said firefighters were already tackling the blazes and water-dropping aircraft would only be deployed if local governors made a request, which they had yet to do.
Ministry official Hadi Daryanto attempted to shift some of the blame onto Malaysia and Singapore, saying their palm oil companies that had invested in Indonesia were also responsible.
"We hope the governments of Malaysia and Singapore will tell their investors to adopt proper measures so we can solve this problem together," he said.
Malaysian Prime Minister Najib Razak said on his Facebook page: "The haze situation in Malaysia is going to worsen in the coming days with winds carrying smoke from hot spots in Sumatra.
"Please reduce outdoor activity and drink a lot of water during this period. Health should remain a number one priority for everyone."
The problem occurs in the dry season as a result of forest fires in the sprawling Indonesian archipelago, some of them deliberately started to clear land for cultivation.
Singapore's Pollutant Standards Index soared to 117 by the early evening on Monday, well past the officially designated "unhealthy" threshold of 100, according to the NEA website.
It said 138 "hotspots" indicating fires were detected on Sumatra on Sunday, and prevailing winds carried smoke over to Singapore.
People with heart and lung disease, those over 65 and children are advised by the NEA to "reduce prolonged or heavy outdoor exertion" even in "moderate" haze conditions, defined as a reading of 51-100.
Singaporean doctor Ong Kian Chung, a respiratory specialist at the Mount Elizabeth Medical Centre, said he expected a surge in patients in the coming days if the haze stays at current levels.
"The usual complaints during haze are throat irritation, eye irritation, cough and difficulty breathing," he said.
Those who have pre-existing respiratory conditions like asthma and chronic bronchitis are more at risk, he said.
Business and air transport have so far not been affected.
Singapore is one of the world's most densely populated countries, with the majority of its 5.3 million people living in high-rise apartment blocks.
Haze was also at unhealthy levels in parts of Malaysia on Monday, particularly in the states of Pahang, Malacca and Terengganu.
Southeast Asia's haze problem hit its worst level in 1997-1998, causing widespread health problems and costing the regional economy billions of dollars as a result of business and air transport disruptions.