The U.N. human rights chief has called for an independent investigation into the police crackdown on striking oil workers in Kazakhstan last December in which at least 15 people died and over 100 were injured after police opened fire on the crowd.

It is extremely damaging to Kazakhstan's reputation to have so much uncertainty hanging over such a serious episode resulting in substantial loss of life, said U.N. High Commissioner for Human Rights, Navi Pillay at a press conference Thursday.

I have recommended to the Government (of Kazakhstan) that the only way to credibly answer these questions once and for all, and draw a line under these tragic events, is to authorize an independent international investigation into the events themselves, their causes and their aftermath.

Oil workers in the Kazakh town of Zhanaozen had been peacefully demonstrating for months, demanding better wages and more equitable labor laws.

The facts revealed so far suggest that there was excessive use of force and abuse of power and that the underlying causes of the situation were fuelled by corruption on the part of the local authorities, Pillay said, criticizing the police response of Dec. 16-17.

Pillay also criticized police and judicial actions in the months following the incident, citing the need for authorities to address allegations of torture to force confessions and the continued use of excessive force.

Last month, 37 oil workers were charged with various crimes in relation to the December clashes, 34 of whom were convicted. While the majority of those charged had their sentences suspended, 13, including a mother of four, were given prison terms of three to seven years.

Allegations of torture and forced confessions do not seem to have been properly investigated, and there are many serious question marks over the fairness of judicial processes, and the conduct of trials, Pillay said.

Pillay concluded her statements by cautioning Kazakh authorities not to take the country's recent economic growth as an excuse to subvert democracy, making an analogy to Tunisia, whose disaffected population ousted President Zine El Abidine Ben Ali in January 2011, kick-starting the Arab Spring.

Kazakhstan has made enormous economic progress since independence, she said. However, I believe the events of December 2011 in Zhanaozen should act as a warning: an economic boom and rapid urban development by themselves are not enough.

Financial prosperity for part of the population should not be sought at the expense of the full spectrum of civil, cultural, economic, political and social human rights for the population at large, she added.

Ignoring this was the mistake made in Tunisia, where very positive economic indicators masked the despair of a population deprived of many of their fundamental human rights.