YANGON, Myanmar — This week began like many others for U Lawi Weng, a reporter in Myanmar with the online magazine The Irrawaddy: on the road with two fellow journalists in a conflict-ridden area near the eastern border with China.

The aim was to “get the facts to readers, as we always do,” said Mr. Lawi Weng’s editor, U Aung Zaw.

But on Monday, Mr. Lawi Weng and two reporters from The Democratic Voice of Burma, another independent news outlet, were detained in Shan State as they emerged from territory controlled by an armed ethnic group. They were charged on Wednesday under a colonial-era “unlawful association” law, which carries a potential jail sentence of up to three years.

The arrests were the latest example of what critics say has been a clampdown on free speech and on the news media during the first 15 months of the first democratically elected government that Myanmar, formerly known as Burma, has had in more than half a century.