A coalition of 30 brands and three global unions have sent a letter to Cambodian Prime Minister Hun Sen calling for an investigation into the recent use of deadly force against striking garment-sector workers.
The letter, dated Friday, urged the government to launch a new process to set minimum wages and to respect the rights of workers and trade unions. The brands also asked for a meeting with Mr. Hun Sen himself.
The letter represented a widening attempt by international brands, including the Walt Disney Co. DIS -0.31% , Wal-Mart Stores Inc., WMT -0.74% Nike Inc. NKE -1.87% andMarks & Spencer Group MKS.LN +1.78% PLC, to use their influence to force change in the Southeast Asian country's largest formal-sector employer and export industry.
A strike launched by garment workers Dec. 24 to press for higher minimum wages ended in bloodshed Jan. 3 when security forces opened fire on a group of demonstrators, killing four and injuring dozens. Government officials have said that the forces acted in the public's interest and to protect the safety of workers.
A day later, security forces forcibly cleared a central rallying site in a park where an opposition party that had received support from the unions was calling for new elections, effectively quelling what had been a strengthening convergence between the opposition and unions on the streets.
Most of the striking workers have since returned to their jobs though a new wage agreement hasn't been reached.
In the days after the crackdown, seven retailers sent a letter signed mostly by compliance officers to the government, factories and unions condemning the violence and calling for a new wage-setting process. They included H&M Hennes & Mauritz HM-B.SK -0.03% AB,Gap Inc., GPS +0.40% Adidas AG ADS.XE +1.02% and Zara's parent company, InditexSA ITX.MC -0.50% . Labor activists were disappointed that more brands hadn't signed on and hadn't emphasized the importance of protecting unions and labor law.
This time, many more megabrands joined in and several chief executives and heads of sourcing signed, indicating that the issue has reached the attention of a broader range of company leaders.

Oum Mean, the Labor Ministry's secretary of state, said he was awaiting instructions from the prime minister before responding to the latest letter.
International brands have grown increasingly concerned that workers' issues remain unresolved and that there could be further turmoil, prompting them to craft the more strongly worded letter dated Friday, a person involved in the process said.
David Welsh, Cambodia program director for the Solidarity Center, a nongovernmental organization affiliated with the U.S.-based AFL-CIO labor group, characterized the first response as weak but commended the brands for the latest letter, and asked that they also tell factories to reinstate fired workers and halt their legal actions against unions.
Ken Loo, secretary-general of the Garment Manufacturers Association in Cambodia, said brands were off base by citing an International Labor Organization Convention that workers should not be penalized for organizing or participating in peaceful strikes, saying that the demonstration "was not peaceful by any measure" because protesters had destroyed factory property.
The strikers had defied government orders to end their walkout. They had called for the minimum wage be raised to $160 a month,—$60 higher than the government's last offer. An alliance of unions and labor activists estimates that a living wage for garment workers would be $283 a month.
Van Sou Ieng, the manufacturers association chairman, said he estimates that the strike cost Cambodia's garment industry roughly $200 million in lost revenue and that orders may decline 20%-30% in 2014.
The garment sector supplies apparel to retailers mainly in the U.S. and European Union. It earned nearly $5.1 billion in the first 11 months of 2013, up 22% from the same period in 2012, according to the Commerce Ministry.
—Chun Han Wong contributed to this article.