Washington, July 21 (Reuters): American officials said they did not believe the US was in imminent danger of an attack similar to today’s second wave of bombings on London’s transport system but were closely monitoring the London events.

Just hours after four small explosions hit London’s bus and underground train network, President George W. Bush said the US would not be frightened by acts of terrorism.

“They have the desire to use their terrorist techniques to frighten us,” Bush said in a speech on trade in which he did not directly mention the bombings. “They understand when they kill in cold blood it ends up on our TV screens. And they’re trying to shake our will.”

“They don’t understand our country though. They don’t understand that when it comes to the defence of universal freedoms, this country won’t be frightened,” he added.

Homeland security officials said the department was closely monitoring the events in London and noted that the US mass transit system was already at a heightened state of alert ? a move taken two weeks ago after the first wave of bombings in central London.

“We have no specific information indicating this type of attack is imminent in the US,” said spokeswoman Katy Montgomery. Defence department spokesman Bryan Whitman said that security had been increased at the Pentagon in a precautionary response to the situation in London, but that there was no evidence of a direct threat to US defence headquarters.

“We have a major metro facility that runs through the Pentagon,” Whitman said referring to a busy underground subway stop beneath the sprawling military headquarters in Virginia near Washington. The increased state of alert on buses, subways and trains means more police are on watch and patrolling stations and platforms.