* Says law should be strengthened to help slaves
* Slavery affects up to 20 pct of population
NOUAKCHOTT, Feb 27 (Reuters) - Mauritania has made progress towards fighting slavery but needs to do more to eliminate it completely, a U.N. human rights expert said on Thursday.
The West African country has outlawed slavery but the Global Slavery Index estimates that about 150,000 people, or 4 percent of the population, remain slaves.
Estimates by other groups are higher at around 20 percent.
"I commend the government of Mauritania for the measures taken since my last mission in 2009 and for its commitment to ending slavery in the country," Gulnara Shahinian, U.N. Special Rapporteur on slavery, said during a four-day visit.
"However, the government still has to turn its pledges into deeds and to take more vigorous measures with a view to eliminating slavery and to fully implement the laws and policies."
Slavery is a historical practice in Mauritania, which straddles black and Arab Africa, and primarily takes the form of chattel slavery whereby adult slaves and their children are the property of their masters.
The Mauritanian government announced a special tribunal to prosecute cases at the end of December.
Still, Shahinian noted the low number of prosecutions and said a 2007 act criminalising the practise should be amended to give better protection for recognised slaves.
She added that the government should also publish recently ratified anti-slavery conventions "in order to raise awareness that all work should be entered into freely and respect the fundamental rights of the human person".
Boubacar Messaoud, president of NGO SOS Esclaves, said that it would take a huge effort to root out slavery as it now forms part of the population's "basic education".
"The Mauritanians are not honest with themselves about slavery. Their traditions and customs are supportive of slavery and unequal. Organisations like the United Nations can help but it's up to each person to lead a personal revolution." (Reporting by Laurent Prieur; Writing by Emma Farge; Editing by Mark Heinrich)