PHNOM PENH (CNN) -- More than 20 striking garment workers were injured this week, including two pregnant women, during clashes with police outside a factory in Kampong Speu Province, Cambodia, a labor leader said.

Roughly 4,000 workers have walked off the job at a factory owned by Sabrina (Cambodia) Garment Manufacturing, which supplies Nike and Lululemon Athletica.

They are demanding a $14 monthly increase, from the $74 they make now, said Say Sokmy, secretary-general of the Free Trade Union of Workers of the Kingdom of Cambodia.

Police used stun batons to disperse strikers on Monday, she said, injured more than twenty.

Both Nike and Lululemon released statements, expressing concern about the incident.

"As outlined in Nike's Code of Conduct, Nike requires its contract manufacturers to respect their employees' rights to freedom of association," the U.S.-based company said in a statement.

It continued: "Workers are employed by the contract factories, not by Nike, and wages and compensation are the responsibility of the factories. It is our understanding that this factory raised its own minimum wage on May 1 and pays above the country's minimum wage."

Vancouver-based Lululemon Athletica said its sustainability teams are monitoring events and are in daily contact with the factory to ensure workers' safety.

"We require all of our vendors to uphold and respect the workers' rights around the freedom of association and collective bargaining as stated in our lululemon Code of Ethics," said Therese Hayes, vice president of sustainability.

"The current situation deeply concerns us and we will continue to be in close contact with our factory partner, and take any immediate actions if necessary."

Both companies stressed that the factory is part of Better Factories Cambodia (BFC), a program established by the International Labor Organization to monitor workplaces and offer advice.

According to BFC, the garment industry employs some 400,000 workers and accounts for $4 billion, or roughly 80%, of Cambodia's total export revenue.

Most of what's made is exported to the European Union and the United States.

Journalist Gregory Pellechi reported from Phnom Penh; CNN's Elizabeth Joseph reported from Hong Kong.