While "both output and new orders increased" across the region, according to IHS Markit, "another severe decline" was recorded in Myanmar "as factories remained closed amid political turmoil."
Hundreds of people have been killed by the military in Myanmar during almost daily protests against its February 1 coup.
The findings were published as part of IHS Markit's monthly Purchasing Managers' Index, which canvasses around 2,100 factory bosses across seven South-east Asian countries.
Elsewhere in the region, the "strongest upturn" in March factory output was in Vietnam, which was one of the few countries to record gross domestic product growth last year as the coronavirus pandemic and related restrictions hammered economies.
Growth was also reported in Indonesia, the Philippines and Singapore in March, while output in Thailand and Malaysia hovered slightly below expansion, according to the index's metrics.
Singapore on Monday reported retail sales in February grew by over 5 per cent, another signal that the region's wealthiest economy, measured per capita, is recovering after a record GDP contraction last year. - dpa