The United States has ordered all "non-essential personnel" out of its embassies in Sudan and Tunisia as violent protests against it continue to rage in the Middle East.
The State Department also warned US citizens not to visit the countries.
Thousands of protesters attacked the US embassy in Sudan's capital Khartoum on Friday, killing three.
In Tunisia crowds smashed up the embassy's entrance building and torched a neighbouring US school. There were two fatalities.
The Sudanese government refused permission for the US to send a team of marines to defend its Khartoum embassy on Saturday - a step which has already been taken in Yemen and in Libya.
Libyan officials blamed "foreigners" today for planning the attack which killed the US ambassador, three embassy staff and up to 10 Libyans, and said they had arrested "about 50" people suspected of involvement.
Attacks on US embassies began last week in Egypt, ostensibly in response to an amateur film made in California which insulted Islam and the prophet Mohammed.
But the way the protests have spread like wildfire throughout the Muslim world - from Morocco to Pakistan and even Malaysia - indicate widespread resentment of the US rooted in its bloody wars in Afghanistan and Iraq and its illegal drone assassinations in Yemen and Pakistan.
And the timing of the Egyptian riot - the 11th anniversary of the September 11 2001 terror attacks - is unlikely to have been a coincidence.
Protests have even spread to the West, with France having to beef up security around the US embassy in Paris after demonstrations at the weekend.
US Defence Secretary Leon Panetta said that he expected the "turmoil" to continue.
The country's ambassador to the United Nations said the violence was caused by those who had "missed out" from the Arab spring uprisings, although Islamists opposed to the US have strengthened their hand in many countries since the movement began.