Kenya has banned flights to and from neighbouring Somalia in what has been described as an anti-terrorist move.
The US embassy in Nairobi is temporarily closed
No explanation has been given.
And it is not clear if the authorities are responding to a specific terrorist threat or if this is part of a broader security operation, our correspondent in Nairobi Andrew Harding reports.
The decision follows Friday's warning of a possible imminent attack on the United States embassy in Nairobi.
Kenyan authorities dismissed the US assessment as "wrong and misleading".
There has been concern for some time that Somalia could be used as a transit point for militants and weapons, our correspondent says.
The country has been without a proper government for over a decade - the US has singled it out as a possible haven for Osama Bin Laden's al-Qaeda network.
The flight ban has chiefly affected traders who fly tons of the mildly narcotic leaf, khat, into Somalia every day.
In recent days, Kenyan police have been involved in a large security operation in a Nairobi suburb which is home to thousands of Somali refugees.
Dozens of people in Eastleigh have been detained - many thought to be suspected criminals and illegal aliens.
The Kenyan authorities are keen to show that they are pulling their weight in the US war against terror, our Nairobi correspondent says.
Tons of khat are flown into Somalia every day
They have been stung by American criticisms that no one has yet been arrested in connection with last November's co-ordinated attacks against an Israeli plane and hotel in Mombassa.
For the past month, the US, Britain and Germany have warned tourists not to visit Kenya because of fears of a new al-Qaeda strike.
The Kenyan Government has urged governments to relax their travel advisories, complaining that the local tourism industry is being devastated.
'Safer than New York'
Kenyan National Security Minister Chris Murungaru told the French AFP news agency on Saturday that the dangers of a fresh attack had been exaggerated by US officials.
"They should know that New York is not any safer than Nairobi," he told AFP.
A suicide bombing blamed on al-Qaeda destroyed the US embassy in Nairobi in 1998, killing 213 people, mostly Kenyans.
The new compound on the edge of the city has only been open for a few months.