SANAA: The United Nations told its staff in Yemen to stay home on Monday, a day after gunmen shot dead a German embassy security guard.
International humanitarian organisations have also advised staff to limit their movements, an aid worker said.
The U.N.'s Department of Safety and Security said in an internal note that all U.N. offices in the country would be closed on Monday and staff should not show up for work.
"Staff should remain at home and not travel. No travel to shops or elsewhere is approved. Tomorrow is uncertain and all staff should be very cautious, it said.
The latest measures follow the temporary closing of Western embassies in August after a security warning over a planned al Qaeda attack.
The German was gunned down while leaving a supermarket in Sanaa's Hadda district, where the embassy is located.
Germany's federal police union said he was a 39-year-old police commissioner who worked for the PSA, a special unit of the federal police responsible for personal protection.
It was the latest in a series of attacks on foreign and local officials in the U.S.-allied state. No one has claimed responsibility.
A German Foreign Ministry spokesman denied reports the killing was part of an attempt to kidnap the ambassador, who had been outside the country but was expected to fly back to Yemen on Monday.
"We are in touch with all relevant authorities in Yemen, we have personnel there dealing with security and this presence might be increased," the spokesman said.
Ambassador Carola Muller-Holtkemper had only recently arrived in Yemen and presented her credentials to Yemeni authorities less than a week ago, the embassy said.
German Foreign Minister Guido Westerwelle said a team of German experts would fly to Yemen to support investigation into the killing. He would not speculate about who was behind it.
"We condemn this gruesome act and are in deep mourning for an engaged and valued foreign office colleague," he said.
The attack unsettled other foreign aid agencies in Yemen.
"Several international organisations have asked their employees to limit their movement and to spend the Eid holiday outside Yemen," the aid worker, who asked not be named, told Reuters.
The Muslim Eid al-Adha holiday is next week.
Yemen-based al Qaeda in the Arabian Peninsula (AQAP) has attacked several Western targets, including airliners, and is believed to have been behind a series of killings of foreign and local officials in the country since 2011.
On Sunday, AQAP said in a statement it was behind an attack on an army base in southeast Yemen last week in protest against the army's cooperation with the United States.
The base contained advanced intelligence equipment supplied by the United States and Germany and that they had been used in the fight against AQAP, it said.
"The killing could be linked to the accusation of Germany as a supporter of operations against al Qaeda," said Mustafa Alani, head of security studies at the Gulf Research Centre in Geneva.
In November last year, gunmen shot dead a Saudi diplomat and his Yemeni bodyguard in Sanaa in an attack believed to be the work of the group. A month earlier, masked gunmen shot dead a Yemeni man who worked in the security office of the U.S. Embassy.
Germany was one of several Western countries which shut their Yemen embassies in early August after a U.S. warning of a possible major militant attack in the Middle East. The mission reopened after two weeks.
Militants took advantage of political chaos in Yemen during the Arab Spring uprising in 2011 to seize control of some towns and surrounding areas in the south of the country.
They were beaten back by Yemeni forces with assistance from the United States and dispersed into smaller groups spread across the south of the country.