Afghan protesters burn a U.S. flag during a protest in Jalalabad province February 24, 2012.
Afghan demonstrators burn a U.S. flag during a protest in Jalalabad province on Friday. Afghan authorities said on Sunday they believe one of the country's police intelligence officers may have been involved in the shooting deaths of two U.S. officers inside the nation's interior ministry a day earlier, prompting NATO to recall all its staff from ministries. REUTERS/Parwiz

(Reuters) - Seven U.S. military trainers were wounded Sunday when a grenade was thrown at their base in northern Afghanistan, police said, as anti-Western fury deepened over the burning of the Koran at a NATO base.

Despite an apology from U.S. President Barack Obama, riots raged across the country for a sixth day on Sunday against the desecration of the Muslim holy book at a NATO air base at Bagram. Some protesters hoisted the white Taliban flag.

The Afghan Interior Ministry identified one of its employees as a suspect in the fatal shooting of two U.S. officers in its headquarters a day earlier, an attack that prompted NATO to recall its staff from ministries.

One civilian was killed, 15 more were wounded and three policemen injured in riots near the NATO base in northern Kunduz province, where the blast that wounded the Americans took place, regional police chief Samihullah Qatra told reporters.

NATO confirmed there had been an explosion outside one of its bases in northern Afghanistan, but declined to comment on casualties.

The protests have killed 30 people and wounded 200, including two other U.S. troops who were shot dead by an Afghan soldier who joined rallies in the country's east.

The Koran burnings could make it far harder for NATO forces to win the trust of the Afghan public as they try to stabilize the country ahead of the withdrawal of foreign combat troops at the end of 2014.

Afghan security sources identified Abdul Saboor, a 25-year-old police intelligence officer, as a suspect in the shooting of the Americans at close range inside the Interior Ministry.

In a statement to media, the ministry said: An employee has been identified as a suspect and he has now fled. The Interior Ministry is trying to arrest the suspected individual.

NATO is supposed to be moving away from a combat role to an advise-and-assist mission as early as next year. That will require NATO to place more staff in Kabul's ministries.

CCTV footage showed that Saboor had access to the Command and Control Centre, tucked deep inside the ministry, where the slain Americans were found, security officials told Reuters on condition of anonymity.

The Taliban took responsibility for the attack, saying it was in retaliation for the burning of the Korans.

Afghan President Hamid Karzai repeated his plea for calm and restraint. It is time to regain and preserve our calm, and not allow our enemies to misuse it, he told reporters, referring to the nationwide violence.