That's all that they are really aiming for, to take out as many Americans as they can

Yvonne Hibbert

She had even thought about calling in sick to work after hearing the news.

"I know it's paranoid. I know they don't want us to be, but you have to be," she told BBC News Online.

Like everyone else, she is haunted by memories of airliners loaded with passengers and jet fuel being flown into the World Trade Center and the Pentagon, killing thousands.

Her colleague, Jennifer Short, said the FBI's action had reawoken a fear of flying - particularly with her husband away on a domestic flight.

The alert named a Yemeni possibly plotting attacks

"I think my husband sensed that - because he called me when he landed. And he doesn't typically do that."

Mrs Short said she had taken special note of the alert for two reasons; it was the first to be issued for a while, and US officials had provided more details than they often do.

The warning was the fourth major one since 11 September - and by far the most specific.

There's not much more we can do. I mean how much higher can we go? We're already on the highest alert

Los Angeles Police Department

Previous alerts have been so vague that some people have questioned their usefulness.

But both women were as one on this point. They definitely wanted to know about possible threats, however lacking in detail.

The information made them more vigilant, they said.

"This morning, I was observing every large truck that I passed by," said Yvonne Hibbert. "I hate to say this, but I'm stereotyping people, looking to see who's driving it, whether it's a foreigner or what."

Despite the alert, police across the United States said it would be more or less business as usual.

We can't stay home every time there's a high alert

Melanie Forrest New York commuter

A spokesman in Los Angeles wondered how much more cautious they could be.

"Ever since the 11 September terrorist attacks, our officers have been on high alert and we continue to remain so," said LAPD spokesman Jason Lee.

"No specific location or time was given for this latest alert, so there's not much more we can do. I mean how much higher can we go? We're already on the highest alert."

'Scary' in New York

New Yorkers, meanwhile, were urged to carry on as normal on Tuesday, despite the attack warning.

"I don't think you should panic," Mayor Michael Bloomberg told them. "If something looks out of the ordinary, exercise common sense, don't go near it and call the authorities."

All I could think about was what happened at the World Trade Center

New York commuter

His efforts were not helped by a scare which closed part of the city's biggest bus station during morning rush hour.

Thousands were evacuated from the Port Authority terminal after a man walked in and told officials he had a bomb.

Officials later said it had been a hoax, and the man was arrested.

Commuters, however, divided into two camps faced with the fears of a new attack.

Many ran from the terminal to safety, and some said they had feared for their lives.

"All I could think about was what happened at the World Trade Center," one woman told the BBC. "I didn't want to be a statistic. It's really very scary."

'Not nervous'

But others were more accepting of security, possibly having become accustomed to measures which are now part of everyday life in New York.

"We can't stay home every time there's a high alert," Melanie Forrest of Bergenfield, New Jersey, told Reuters.

Her views were echoed by another commuter, who said it was better to be safe than sorry.

"It doesn't make me nervous," he told the BBC. "I believe the city is doing everything that's necessary to keep the city secure and safe.

"Inconvenience is something we have to deal with."