Norway's embassy in Iran was also targeted

Condoleezza Rice said both countries had used the opportunity to incite violence and exploit Muslim anger.

The accusation came after Western embassies came under attack in Iran, Syria and Lebanon.

Meanwhile the Danish government severed ties with local clerics, saying they had stirred up anti-Danish feeling.

Radical Danish imams made two visits to the Middle East in December and January to complain about the 12 cartoons to political and religious leaders.

The cartoons, first published in a Danish newspaper in September, show the Muslim Prophet Muhammad in a variety of humorous or satirical situations.

They include images of the Prophet carrying a lit bomb on his head and brandishing a sword.

But one of the clerics, Ahmed Akari, told the BBC the imams carried three extra caricatures of Muhammad that were far more inflammatory than the original 12, and were believed to have been drawn by extremists as part of a hatemail campaign.

Young Iranians give their views on the cartoon issue In pictures

Mr Akari said the imams had added them to their dossier to demonstrate the kind of attitudes that Muslims were facing in Denmark.

He insisted they were not intended to aggravate reaction in the Middle East.

Protests against the cartoons have continued, with four killed in an Afghan demonstration.

The deaths - at a protest by about 400 people in the town of Qalat - bring to 12 the number of people killed in Afghan protests over the cartoons in recent days.

Afghanistan's top council of Muslim clerics has called for an end to several days of demonstrations.

In other developments:

• French magazine Charlie Hebdo becomes the latest to print the cartoons

• Hackers attack hundreds of Danish websites, posting pro-Islamic messages condemning publication of the images

• Several hundred people march on the Italian embassy in the Bangladeshi capital Dhaka, but are blocked by police

• International observers leave their mission in the West Bank town of Hebron, following an attack by hundreds of Palestinian protesters.

'Sentiments inflamed'

Speaking at a joint news conference with Israeli Foreign Minister Tzipi Livni, Ms Rice said some countries were behaving responsibly with regard to the row but that others "have also used this opportunity to incite violence."

CARTOON ROW 30 Sept 2005: Danish paper publishes cartoons 20 Oct: Muslim ambassadors complain to Danish PM 10 Jan 2006: Norwegian publication reprints cartoons 30 Jan: Gunmen raid EU's Gaza office demanding apology 1 Feb: Papers in France, Germany, Italy and Spain reprint cartoons 4 Feb: Danish and Norwegian embassies in Damascus attacked 5 Feb: Protesters set alight Danish embassy in Beirut 6-7 Feb: At least eight killed in protests in Afghanistan In pictures: Cartoon protests How can row be resolved?

"I don't have any doubt that ... Iran and Syria have gone out of their way to inflame sentiments and to use this to their own purposes. And the world ought to call them on it," she said.

On Wednesday, about 200 Iranian demonstrators attacked the UK embassy in Tehran, but were prevented by police from forcing entry to the building.

The Danish and Norwegian embassies in Iran have also been attacked, while those in Beirut and Damascus were set on fire at the weekend.

There have also been violent clashes across the Muslim world since the end of January when some European newspapers republished the cartoons.

Arab and Muslim ambassadors complained to the Danish government about the cartoons last October - calling for Muslim feelings to be respected - but were told that there was nothing the government could do.

In a separate statement, US President George W Bush urged governments to prevent attacks on diplomatic missions.

"I call upon the governments around the world to stop the violence, to be respectful, to protect property, protect the lives of innocent diplomats who are serving their countries overseas," he said.

French President Jacques Chirac, however, focused on the European media, condemning decisions to republish the cartoons as an "overt provocation".