Nobel Peace Prize winner Aung San Suu Kyi will defend genocide allegations against Myanmar's military on Wednesday amid accusations of mass killings, rape, and expulsion of the Rohingya Muslim minority.

The Gambia, a small West African country, launched the case against Myanmar at the International Court of Justice (ICJ), the United Nations's highest court, alleging it violated the 1948 Genocide Convention.

Aung San Suu Kyi - once a human rights icon who fought against the powerful military for democracy - shocked critics and galvanised supporters at home by travelling to The Hague to head her country's delegation. Her office said she was going to "defend the national interest".

She listened impassively on Tuesday as lawyers for The Gambia detailed graphic testimony of suffering of Rohingya at the hands of Myanmar's security forces, including gang rape, torture, and murder.

"It was very important to see her have to sit inches away from people who were describing - in really painfully excruciating detail - all the horrible crimes of the Burmese military that happened on her watch," Brad Adams, of New York-based Human Rights Watch (HRW), told Al Jazeera. 

Although Aung San Suu Kyi has not revealed details of her government's defence, she and her legal team are expected to argue that the court lacks jurisdiction and that no genocide has taken place in Myanmar.

In three days of hearings this week, judges are hearing the first phase of the case: The Gambia's request for "provisional measures" - the equivalent of a restraining order - against Myanmar to protect the Rohingya population until the case is heard in full.

The Gambia has argued that it is every country's duty under the 1948 Convention to prevent a genocide from taking place. It has political support from the Organisation of Islamic Cooperation, as well as several Western nations including Canada and the Netherlands.

Gambian Minister of Justice Abubacarr Tambadou said outside the court it would be "extremely disappointing" if Aung San Suu Kyi repeated her previous denials of wrongdoing by Myanmar.

More than 730,000 Rohingya fled Myanmar after the military launched a crackdown in the country's western Rakhine state in August 2017. Most now live in crowded refugee camps in Bangladesh.

Myanmar argues that the military's "clearance operations" in Rakhine were a justifiable response to acts of "terrorism" by rebels and says its soldiers acted appropriately.

'Hugely important'

Although a UN fact-finding mission found "the gravest crimes under international law" were committed in Myanmar and called for genocide trials, no court has weighed the evidence.

According to some analysts, the continuing trial itself represents a huge landmark.

"The final judgement can take a long time [of up to five years], but for victims and their communities, it's an incredible moment," Antonia Mulvey, founder of Legal Action Worldwide and an expert on international human rights, told Al Jazeera.

"This sends a very strong message to the Rohingya that the international community is watching and listening to them."

Speaking from Cox's Bazar in Bangladesh, the site of the largest Rohingya refugee camp in the world, Al Jazeera's Stefanie Dekker reported that the trial was "hugely important" for the Rohingya.

Dekker said people at the camp were aware of the trial and were trying to get news of it.

"The internet had been cut for the last couple of months when a large protest took place at the two-year anniversary ... but today, for some reason, 3G is back on and people are able to access the news."

US slaps sanctions

In another blow to the Myanmar government, the United States stiffened sanctions against the Southeast Asian nation's army chief, Min Aung Hlaing, and three other senior commanders on Tuesday over the killings. "The United States will not tolerate torture, kidnapping, sexual violence, murder or brutality against innocent civilians," Treasury Secretary Steven Mnuchin said in a statement.

The new sanctions, which coincided with the UN's International Human Rights Day, freeze any US assets held by those targeted and prohibits Americans from doing business with the named individuals.

John Sifton, Asia advocacy director for HRW, called it a welcome but overdue step, saying "better late than never".

"It is unfortunate the decision took so long. The crimes in question were incredibly serious," he said.

Al Jazeera