Five persons were killed and two were injured in two Northern Virginia car accidents yesterday, police said.

A Virginia State Police spokeswoman said Marzena Tobias, 19, and her mother-in-law, Jo Ann Tobias, 52, both of Cardinal Glen Circle in Sterling, were apparently killed instantly about 12:30 p.m. when their southbound Honda crossed the center line on Rte. 28 just north of the Dulles Toll Road in Loudoun County. Their vehicle slammed head-on into a cement truck and burst into flames, police said.

The driver of the truck, Lynval Anthony Allen of 6900 High View Ter. in Hyattsville, was reported in fair condition at Reston Hospital, officials said.

Rte. 28, a major road through Loudoun and western Fairfax counties, was closed for about three hours, according to state police Sgt. M.E. Hoover.

The truck is owned by a Herndon firm, Hoover said. It was not known if any of the victims was wearing seat belts.

In another accident in Manassas at Rte. 234 and Lucasville Road, three men were killed and one was injured when the white 1979 Fiat they were in struck a tractor-trailer at 6:18 p.m., Prince William County police said.

Police said the driver of the Fiat made a left turn from the wrong lane on Lucasville Road and pulled out in front of a tractor-trailer that was southbound on Rte. 234, also known as Dumfries Road.

Two men in the Fiat were pronounced dead at the scene, police said.

One passenger in the Fiat was taken to Prince William Hospital, where he died shortly afterward, and a fourth passenger was undergoing surgery last night at Fairfax Hospital.

The names of the victims were not released until relatives could be notified.

It was not known if any of the victims was wearing a seat belt.

The driver of the tractor-trailer, J. Donald Heer, of Pennsylvania, was not injured, police said.