LONDON (Reuters) - British anti-terror detectives were hunting on Monday for more members of a suspected militant cell who rammed a burning jeep into a Scottish airport and tried to detonate two car bombs in central London.

Cars and other vehicles were banned from directly approaching airports and security measures were stepped up across the country as authorities set the threat level at "critical", meaning the possibility of an attack is "imminent".

Ministers said "good progress" was being made in the investigation and police sources said a manhunt was under way for an unspecified number of suspects after five people were arrested at the weekend. All five detained were thought to be foreigners and were arrested under anti-terrorism laws.

Home Secretary (interior minister) Jacqui Smith said Britain was facing a "serious and sustained threat of terrorism" and urged the public to remain on alert.

"We are not going to let this terrorism intimidate us," she told Sky News. "But ... obviously we need the increased security measures that are in place and we need the public to carry on being as vigilant as possible."

Police declined to identify any of the people under arrest, but British newspapers said two of those in custody were doctors -- one an Iranian doctor who worked at North Staffordshire Hospital in central England. A spokeswoman at the hospital declined to comment.

On Saturday, police arrested the passenger and badly-burned driver of a Jeep Cherokee who had rammed the vehicle into the entrance of Glasgow's airport, causing a huge fireball.

The attack came 36 hours after police in London found two Mercedes car bombs packed with fuel canisters, propane tanks and nails parked near a crowded nightclub in the capital's busy theatre district.

The plots, which police believe are linked, raised the specter of attacks on London transport two years ago that killed 52 commuters.

OTHER ARRESTS

The other arrests included a 26-year-old man and a 27-year-old woman seized on a motorway in northern England on Saturday, and a 26-year-old man in Liverpool on Sunday.

"The investigation is continuing", a police spokesman said, adding that two of the suspects had been taken to London for questioning by anti-terror detectives on Monday.

The head of the Scotland's administration, First Minister Alex Salmond, said he and Prime Minister Gordon Brown had been co-operating closely over the security alert.

"These are challenges we face north and south of the border and the prime minister and myself have been at one in making sure that the country won't be intimidated or stopped from going about its normal business by terrorist outrages," Salmond told BBC radio.

Brown, who has been in the job less than a week after he took over from Tony Blair on Wednesday, said on Sunday he believed those behind the botched attacks were associated with al Qaeda.

Police and ministers said protective security measures would be stepped up across Britain, particularly at transport hubs.

"We will obviously be stepping up high visibility policing and we are working with other agencies, including British transport police, to help ensure security," a spokesman for London's Metropolitan police said.

He said extra police officers would be patrolling and carrying out more searches in areas where there were lots of people, including rail stations and airports.