"Given the uncertainty and volatility of the current situation, US citizens in areas where events could cause anti-American violence are strongly urged to limit their travel outside of their homes and hotels and avoid mass gatherings and demonstrations," the State Department said in a statement.

President Barack Obama warned Americans Sunday night to remain vigilant even after the killing of al Qaeda leader Osama bin Laden and while there are no known credible threats, the risk of attacks remains.

"There is no doubt that al Qaeda will continue to pursue attacks against us. We must and we will remain vigilant at home and abroad," Obama said in a late-night televised statement announcing that US forces had killed bin Laden.

DHS and FBI officials had no immediate comment about the risk of attacks or any new threats. While bin Laden was seen as the leader of al Qaeda, because he was in hiding from US forces he was reduced more to a figurehead, experts said.

Meanwhile affiliates of his militant group have taken the lead in launching attacks. Most attacks against US interests have been by a Yemeni affiliate, al Qaeda in the Arabian Peninsula (AQAP).

The group has claimed responsibility for trying in October to send bombs packed in toner cartridges aboard cargo planes bound for the United States. They were intercepted and failed to detonate.

AQAP also backed an attempt on Christmas Day 2009 by a Nigerian manwho tried but failed to detonate a bomb hidden in his underwear while aboard a US commercial flight as it approached Detroit from Amsterdam.

"This doesn't end the terrorist threat to the United States, but it's the end of a key chapter to the War of Terror," said Juan Zarate, who served as deputy national security adviser for combating terrorism during George W. Bush's presidency.

"There may be a spike of threats initially, and there are other elements of the al Qaeda network who remain dangerous," said Zarate, now a senior adviser at the Center for Strategic and International Studies.