The sniper who has killed nine people in the area around the US capital Washington has issued a warning that children are under specific threat, according to police.

The warning comes as officers made a fresh appeal for the sniper to contact them over demands made in a message left at the scene of the last confirmed shooting, on Saturday night.

It is rumoured that the note contained a demand for a large sum of money.

Chief Moose: Resumes dialogue with the sniper through the media

Montgomery County Police Chief Charles Moose read out part of a message the police believe was from the sniper.

It read: "Your children are not safe anywhere or at anytime."

Mr Moose urged the sniper to make contact with law enforcement officials about "options".

"We have researched the options you stated and we have determined it is not possible electronically to comply in the manner that you requested," he said at a news conference.

"However, we will remain open and ready to talk to you about the options you have mentioned.

"It is important to do this without anyone else getting hurt."

He suggested that a telephone line, a mailbox or some other method could be used to maintain contact.

Schools on alert

Many children are expected to be kept away from school in the Washington area on Wednesday following the threat.

	:	Trail of terror
Map showing location of recent attacks

Schools in the region have reportedly been briefed by police regarding safety. Some have closed, while others have banned all outdoor activities.

Hannah Matthews, a student in Maryland, told BBC News Online that children were feeling afraid.

"I don't feel safe at all. We are in our third week of lock-down at our school. I feel as if I am in prison," she said.

Earlier on Tuesday, Conrad Johnson, a 35-year-old bus driver, was hit in the chest by a single bullet. He died hours later in hospital.

Mr Johnson was shot as he stood on the steps of his stationary vehicle in the Aspen Hill area of Montgomery County, on the outskirts of Washington.

It has not yet been officially confirmed that his death is linked to the sniper shootings which have terrorised America since early October, killing nine people and seriously injuring three.

Fear spreads

A series of shootings in the area over the past three weeks have been attributed to the sniper, spreading fear among local residents.

I've lived in northern Virginia my entire life, and there is an atmosphere of fear Patrick Murphy, Fairfax County Virginia, USA

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On Tuesday at an interfaith service the Catholic Archbishop of Washington called for all Americans to pray for the killings to end, in an apparent attempt to bring comfort to residents traumatised by the murders.

Tuesday's shooting took place on a main road, Connecticut Avenue, about 15 miles (25 kilometres) north of the US capital just before 0600 local time (1000 GMT).

The road was cordoned off by police, causing heavy traffic jams during rush hour, and surveillance helicopters circled overhead.

BBC Washington correspondent Justin Webb says the police search is becoming increasingly frantic as, once again, the killer appears to have got away.

The killer has often struck near main roads to make a getaway easier. The scene is close to those of the first six shootings.

But the incident took place while it was still dark, much earlier in the day than any previous shootings.

The bus was parked at the side of the road where it was being cleaned and its next route planned.

The scene was a residential area rather than a commercial area, near woodland which would provide ideal cover for a sniper.