Kabul, Afghanistan ? International peacekeepers clashed Tuesday with Afghans protesting drawings of the Prophet Muhammad, leaving three demonstrators dead and prompting NATO to send reinforcements to a remote northern city.

Senior Afghan officials said al-Qaida and the Taliban could be exploiting anger over the cartoons to incite violence, which spread to at least six cities in a second day of bloody unrest in Afghanistan.

Demonstrations rumbled on around the Muslim world, and the political repercussions deepened, with Iran suspending all trade and economic ties with Denmark, where the drawings were first published. The Danish prime minister called the protests a global crisis and appealed for calm.

In a new turn, a prominent Iranian newspaper, Hamshahri, invited artists to enter a Holocaust cartoon competition, saying it wanted to see whether freedom of expression – the banner under which many Western publications reprinted the prophet drawings – also applied to Holocaust images.

The drawings – including one depicting the prophet wearing a turban shaped as a bomb – have touched a raw nerve among Muslims. Islam is interpreted to forbid any illustrations of Muhammad for fear they could lead to idolatry.

Violence has escalated sharply in Afghanistan this week, and seven people have died in demonstrations during the past two days. Protests, sometimes involving armed men, have been directed at foreign and Afghan government targets – fueling suspicions there’s more behind the unrest than religious sensitivities.

“It’s an incredibly emotive issue. This is something that really upset Afghans,” said Joanna Nathan, senior Afghanistan analyst at the International Crisis Group, a Brussels-based research institute. “But it is also being used to agitate and motivate the crowds by those against the government and foreign forces” in Afghanistan.

On Monday, about 2,000 protesters tried to storm the main U.S. military base at Bagram. A top local official said al-Qaida and Taliban militants incited the crowd.