U.S. President Barack Obama has issued a security alert in the country and travel alert worldwide for Americans, expecting retaliatory attacks from terrorist organizations worldwide, especially Al Qaeda, after news spread that dreaded terrorist leader Osama bin Laden was killed in a targeted U.S. operation in Abbottabad, Pakistan.

Dreaded terrorist bin Laden was shot dead on Sunday by U.S. forces led by CIA.

The U.S. forces attacked the terrorist, who was holed up in a multi-million dollar mansion that had 12ft-18ft walls and was eight times larger than other homes in the area. Surprisingly, it had no telephone or internet connection.

Together with bin Laden, his two sons, two couriers and a woman believed to be his personal shield, were killed.

A senior U.S. official said the mansion was under observation for several months and the operation was a kill operation i.e. they were not given orders to take bin Laden alive.

Shortly after bin Laden was killed, Obama appeared on television and said justice has been done.

At my direction, the United States launched a targeted operation against that compound in Abbottabad, Pakistan. A small team of Americans carried out the operation with extraordinary courage and capability. No Americans were harmed. They took care to avoid civilian casualties. After a firefight, they killed Osama bin Laden and took custody of his body.

For over two decades, bin Laden has been al Qaeda's leader and symbol, and has continued to plot attacks against our country and our friends and allies. The death of bin Laden marks the most significant achievement to date in our nation's effort to defeat al Qaeda.

Yet his death does not mark the end of our effort. There's no doubt that al Qaeda will continue to pursue attacks against us. We must - and we will - remain vigilant at home and abroad, the U.S. president said.

A Pakistan Taliban spokesman said they would carry out revenge attacks against American and Pakistani governments and their security forces, prompting the U.S. government to sound a security alert.

In New York, subway stations and bus terminals were teeming with police officers. Armed members of the NYPD have also secured John F. Kennedy airport.

Police officers could also be seen patrolling outside mosques, churches and synagogues, in Time Square and in the vicinity of the World Trade Center site.

Elsewhere in Washington and other states, police forces, paramilitary forces and commandos were put on heightened alert and secured transit hubs, hotels and government buildings.

U.S. embassies worldwide, especially in Pakistan, Saudi Arabia, India, Indonesia and Philippines, have also been put on high alert and American travelers worldwide have been sounded the travel alert.

Given the uncertainty and volatility of the current situation, U.S. 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.

Americans abroad have been urged to keep in regular contact with their friends and family back home. They have also encouraged to register their travel with the Department of State's Smart Traveler Enrollment Program (www.travel.state.gov).

The Department of Homeland Security and the FBI are working overtime to remind security forces to be extra vigilant and brace for retaliatory attacks from terrorist organizations.

Though bin Laden is dead, al-Qaeda is not, CIA Director Leon E. Panetta said in a message to employees. The terrorists almost certainly will attempt to avenge him, and we must - and will - remain vigilant and resolute.

Up-to-date information on security can also be obtained by calling 1-888-407-4747 toll-free in the United States and Canada or, for callers outside the United States and Canada, a regular toll line at 1-202-501-4444.

Here's a skinny on the security situation in the United States -