Voting in Myanmar's by-elections is under way in a first test of the popularity of Aung San Suu Kyi's National League for Democracy (NLD) since it formed the government a year ago

Saturday marks the first time the country has gone to polls since NLD swept to power, in an early indication of views on her leadership amid increased fighting with armed ethnic groups and slower economic growth.

Hundreds of voters lined up outside polling stations on the outskirts of Myanmar's commercial capital Yangon, though the scene lacked the fanfare and enthusiasm that marked the historic 2015 polls.

The by-elections will fill 19 vacant seats in the national and regional parliaments at a time when Aung San Suu Kyi is struggling to match the sky-high expectations.

In a televised address earlier this week, she acknowledged the public's frustration with the slow pace of reforms and development.

But she also reiterated her top priority of ending the ethnic conflicts that have kept Myanmar in a state of near-perpetual civil war.

While the outcome of the by-elections will not affect the balance of power within the parliament where the NLD enjoys a large majority, it offers a chance to gauge the popularity of the administration in a country where nationwide public polls are not available.

Win Htein, one of the NLD's top leaders, said the party faced language barriers and problems with armed groups in the Shan state districts being contested. Fighting in some of those areas has intensified in recent months.

"We are still improving in Shan state. The local people don't understand Burmese, so we have to translate our policies into the Shan language," he said.

Major rebel armies engaged in clashes with the military in areas including the Shan state have refused to actively participate in Aung San Suu Kyi's peace process.

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Several conflicts have reignited since Aung San Suu Kyi took office, displacing an estimated 160,000 more people, according to the UN.

Nobel Peace Prize winner Aung San Suu Kyi is also facing international criticism for her government's handling of a crisis in the Muslim-majority Rakhine region, where soldiers have blocked access for aid workers and are accused of raping and killing civilians.

The UN Human Rights Council agreed to send a fact-finding mission to Myanmar to investigate alleged abuses by security forces against Rohingya Muslims.

Tens of thousands of people have fled Rakhine since the military began a security operation last October in response to what it says was an attack by armed Rohingya men on border posts, in which nine police officers were killed.

A February UN report said the operation targeting the Rohingya involved mass rapes and killings, possibly amounting to crimes against humanity.

Over two million voters, less than five percent of the country's population, can vote in the by-elections where seats in eight states and regions across the country are up for grabs.

The by-elections will fill seats largely vacated by incumbents who joined the government in ministerial posts.

Some seats that were excluded from previous elections due to fighting with ethnic armed groups are also up for grabs.

In strife-torn Rakhine state on Myanmar's western coastline, the party will face a strong challenge not only from the local ethnic minority Arakan National Party, but also the military-backed USDP.

The USDP led the transitional government that took over from the military government in 2011, but was trounced in elections four years later that swept the NLD to power.

To the south in Mon, the NLD is facing a backlash over the naming of a new bridge after Aung San Suu Kyi's father that many see as a symbol of the party's disregard for minorities.