Several mega cities in the US, including New York, Los Angeles, Chicago and Washington, have been put on alert after two explosions in Boston that left two dead, besides injuring many.
Security in and around all critical establishments has been tightened and according to a CNN report, flight movements over the bomb blast site in Boston have been banned.
Although both President Barack Obama and the FBI have chosen not to jump into any conclusions about the nature of the explosions, Senator Dianna Feinstein, who is chairwoman of the powerful Senate Intelligence Committee, termed it a terrorist attack, based on her understanding.
Media reports said the federal investigating agencies and law enforcement bodies have increased their check on all outgoing international flights, indicating that they might be looking into the possibility of a foreign link to the attack that sent shock waves to the entire nation.
Security of the White House was stepped up soon after the bomb blasts in Boston.
"Out of an abundance of caution, we have expanded our security perimeter at the White House complex. It is not unusual to expand or contract these security perimeters," Secret Service spokesman Edwin Donovan said.
Los Angeles Mayor, Antonio R Villaraigosa said the law enforcement agencies have been put on alert.
"We are working closely with our law enforcement partners to monitor the situation and stand ready to aid Boston in any way possible. The LAPD and other public safety agencies will remain vigilant at all public and sporting events in Los Angeles," he said.
New York Mayor Michael Bloomberg also said the security in the city has been stepped up following the Boston blasts.
"I have spoken to Police Commissioner Ray Kelly, and the NYPD has stepped up security at strategic locations and critical infrastructure, including our subways. Some of the security steps we are taking may be noticeable, including deployment of Critical Response Vehicles and additional police personnel, and others will not be," Bloomberg said.
"We have 1,000 members of the NYPD assigned to counter-terrorism duties, and they, along with the entire NYPD and the investments we have made in counter-terrorism infrastructure, are being fully mobilised to protect our city," he said.
Chicago Mayor Rahm Emanuel, in his statement, hailed the patriotism and professionalism of the people and public servants for their quick response to the incidents.
"While the details of today's tragedy are still unclear, one thing was immediately known -- the patriotism and professionalism of public servants and first responders," Emanuel said.
In Miami, the police said they were increasing security at critical infrastructure sites.
Secretary of State John Kerry who was on his way to Chicago from his 10-day foreign trip called his daughter Vanessa, who has run the Boston Marathon in the past, a senior State Department official said.
Flight movements over the bomb blast site in Boston have been banned, CNN reported.
