The United States issued a worldwide travel alert warning its nationals of the "potential for retaliation" after two key Al-Qaeda figures were killed in an air raid in Yemen.
The State Department notice came one day after the killing of US-born Islamic cleric Anwar al-Awlaqi, who President Barack Obama said was the external operations leader of Al-Qaeda in the Arabian Peninsula (AQAP).
Also killed in the air strike was Pakistani-American Samir Khan, who was the editor of Al-Qaeda's English-language magazine "Inspire."
"Awlaqi's standing as a preeminent English-language advocate of violence could potentially trigger anti-American acts worldwide to avenge his death," the State Department said in its alert.
"In the past Awlaqi and other members of AQAP have called for attacks against the United States, US citizens and US interests," it added.
Yemen's defense ministry said Awlaqi was killed early Friday in the air strike in Marib province, a hotbed of Al-Qaeda activity. A man wounded in the attack was quoted as saying a total of seven people were killed.
New York City police commissioner Ray Kelly said Friday that his force was on alert following the killing of Awlaqi, acknowledging his sympathizers might seek revenge.
"We know al-Awlaqi had followers in the United States, including New York City, and for that reason we remain alert to the possibility that someone might want to avenge his death," Kelly said in a statement.
The FBI has also warned about possible revenge attacks by supporters of Awlaqi, who was wanted over anti-US attacks and whose death was hailed by Obama as a "major blow" to terrorists.
It was "a notice to law enforcement to maintain a heightened awareness," said Kathy Wright, an FBI spokeswoman.
"A law enforcement bulletin has gone out to maintain heightened awareness as a result of the deaths that occurred overseas."
In April 2010, a US official said the Obama administration had authorized the targeted killing of Awlaqi after intelligence agencies concluded he was directly involved in anti-US plots.
The White House on Friday dodged questions about the circumstances surrounding the killing, though US Defense Secretary Leon Panetta thanked the Yemenis for their "assistance" in the successful targeting of Awlaqi.
Obama said Awlaqi's killing was a tribute to the US intelligence community and to Yemen's cooperation with the United States in a common anti-terror campaign, while warning that, though "weakened," AQAP is still "dangerous."
AQAP has taken advantage of nearly nine months of sweeping unrest against President Ali Abdullah Saleh to bolster its presence in the restive south and east of Yemen, launching regular attacks.
On Friday, AQAP issued a new claim of responsibility for deadly attacks in the south in a statement received by AFP, but made no mention of Awlaqi's death.
Five Yemeni soldiers were killed on Saturday in an attack on the southern city of Zinjibar during an operation aimed at rooting out militants linked to Al-Qaeda, a medical source said.