Four people were shot dead by Afghan security forces on Saturday as protests over the burnings of the Muslim holy book at a NATO base erupted for a fifth day, with an attempt by demonstrators to bombard a U.N. compound in the north.

The burning of the Korans at the Bagram compound this week has deepened public mistrust of NATO forces struggling to stabilise Afghanistan before foreign combat troops withdraw by end-2014.

Despite an apology from U.S. President Barack Obama and a call for restraint from Afghan leader Hamid Karzai, thousands took to the streets after 12 people were killed and dozens wounded on Friday, the bloodiest day yet in demonstrations.

Protests were raging in the restive northern Kunduz province, where three protesters were shot dead and 50 wounded, said health official Saad Mukhtar.

Hundreds of protesters tried to overrun a compound there housing workers from the U.N. Assistance Mission in Afghanistan (UNAMA), but were held back by police, Interior Ministry spokesman Sediq Sediqqi said.

A similar incident occurred in April last year when protesters angry over the burning of Korans by an obscure pastor in the United States stormed a U.N. compound in northern Balkh province, killing seven.

A protester was shot dead in Logar province south of Kabul on Saturday after hundreds of protesters, many chanting Death to America! - a slogan heard at protests throughout this week -- charged at police, local officials said. Two people were wounded.

Twenty people were wounded when demonstrators hurled stones in eastern Laghman province, health official Abdul Qayumi said.

The capital, Kabul, was calm, with police and security forces deployed across the city.

Muslims consider the Koran to be the literal word of God and treat each copy with deep reverence. Desecration is considered one of the worst forms of blasphemy.

The Koran burnings underscore the deep cultural divide that still exists more than 10 years after U.S. troops invaded to oust the Taliban and have deepened public mistrust of the West.

The protests could dent plans for a strategic pact that Washington is considering with Kabul, which would allow a sharply reduced number of Western troops to stay in the country, well beyond their combat exit deadline.

(Writing by Amie Ferris-Rotman, editing by Ron Popeski)