The British newspapers "The Daily Telegraph" and "The Times" quote U.S. military spokeswoman Cindy Dorfner as saying family members of the American personnel were also being encouraged to stay away from London because of security concerns.
London authorities are meanwhile continuing forensic work to identify the victims and hunt for clues to who carried out the four bombings on 7 July that killed at least 52 people and injured some 700 others.
In Washington, the U.S. Senate held a minute of silence on 11 July to honor the victims of the London bomb attacks.
The Senate also voted unanimously, 76-0, for a measure expressing the United States' sympathy for the British people.
On 11 July, British Prime Minister Tony Blair, addressing the House of Commons in London, said that "Islamic extremist terrorists" probably carried out the 7 July attacks. Blair pledged that authorities would not rest until those who carried out and planned the bombings were brought to justice.
"We will pursue those responsible, not just the perpetrators, but the planners of this outrage, wherever they are, and we will not rest until they are identified and, as far as humanly possible, brought to justice," Blair said.
Blair also praised the resilience and spirit of Londoners, who returned to work on 11 July using buses and underground trains like those that were targeted in the attacks.
U.S. President George W. Bush said the bombings showed the need for what he called the "civilized world" to keep fighting terrorists until they are defeated.
"These kind of people who blow up subways and buses are not people you can negotiate with or reason with or appease. In the face of such adversaries, there is only one course of action: we will continue to take the fight to the enemy, and we will fight until this enemy is defeated," Bush said.
Bush, in a speech to the FBI training academy near Washington, said the only way to defeat Islamic militants is to "advance the cause of freedom" in Iraq and elsewhere in the Middle East.
Hunt Goes On For Clues In London Bombings
U.K. Struggles To Confront Security Threat