The United States said it was "deeply concerned" after the outbreak of violence in western Kazakhstan and urged authorities to reopen communication throughout the country, AFP reports.

The U.S. State Department also called for justice against anyone found to be responsible for criminal acts after the violence claimed at least 15 lives in an oil-producing region.

"The United States is deeply concerned by the recent outbreak of violence in western Kazakhstan," State Department spokeswoman Victoria Nuland said, urging "restraint by all parties."

"We are concerned by reports that some forms of communication were disabled in the wake of these protests and urge the government of Kazakhstan to quickly and fully restore open lines of communication throughout the country," she said.

Kazakh President Nursultan Nazarbayev has declared a 20-day state of emergency in the western city of Aktau after unprecedented riots, a blow to a government that prides itself on attracting foreign investors.