UN Secretary-general Ban Ki- moon is "concerned" about the escalation of violent clashes between security forces and protesters which have left at least 21 dead in Tunisia, and called for "restraint," said a UN spokesperson here on Thursday.

The North African country has been witnessing clashes for more than three weeks, with protesters angry about soaring food and fuel prices, and high unemployment rates.

"The secretary-general calls for restraint and urges all parties to seek to resolve differences through dialogue," Martin Nesirky, the UN spokesperson, told reporters here at a daily press conference.

"He underscores the importance of full respect for freedom of expression," Nesirky said.

UN High Commissioner for Human Rights Navi Pillay on Wednesday urged the government of Tunisia to ensure that its security forces cease using excessive force against demonstrators, and to launch transparent and credible investigations into the deaths that have taken place.

"Whatever the precise total, I am extremely concerned about the very high number of people killed in Tunisia in recent weeks," Pillay said.

"It is essential that basic international human rights norms and guidelines governing the use of firearms are urgently and strictly adhered to," Pillay said in a statement.

On Wednesday, the Tunisian Interior Ministry announced curfew in Grand Tunis region starting from 8:00 p.m. (1900 GMT) Wednesday to 5:30 a.m. (0430 GMT) Thursday, after unrest spread to the capital.