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

The Department of Homeland Security (DHS) and the FBI have not issued any warning of a credible or imminent threat in the wake of news that bin Laden was killed in Pakistan, but security will likely be ramped up to guard against possible retaliation.

"There is no doubt that al-Qaida 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-Qaida 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 man who 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-Qaida network who remain dangerous," said Zarate, now a senior adviser at the Center for Strategic and International Studies.