MANILA, JUNE 6 -- Early in March, Philippine Constabulary Lt. Col. Benjamin Casabar and his driver were waiting at a traffic light at a busy Quezon City intersection, when three gunmen opened fire from the median strip. As onlookers watched in horror, Casabar fell dead on the seat of his jeep. His driver died at a nearby hospital.

Casabar and his driver were among the first victims of what is believed to be a new and deadly phase in the Philippines' 18-year-old communist insurgency: an intensified urban assassination campaign by highly trained and mobile hit men -- known as "sparrow units."

The sparrows move in small groups, usually of three men, and operate in daylight on busy streets -- allowing them to quickly disappear into crowds. They often make their getaways on motor bikes, or by commandeering passing cars. Their targets: high-ranking policemen, military officers and other senior law enforcement officials.

Armed Forces Chief of Staff Gen. Fidel V. Ramos said today that authorities have confirmed at least seven sparrow assassinations over the past three months. The government suspects many more. At least 40 policemen, soldiers and security guards have been assassinated in Manila this year, including three top police officers in a 36-hour period this week, and some of these may also have been victims of the sparrows.

While the guerrilla war has raged unabated in the countryside, claiming 10 lives daily since February's collapse of a cease-fire, the urban sparrows have emerged as one of the deadliest, most effective and highly visible weapons of the communists. Military officials say the sparrows are ruthlessly efficient when operating on crowded city streets, and are virtually impossible to apprehend.

It is even difficult for military officials to determine which murders are in fact the work of sparrows, or are robberies, personal vendettas or killings by crime syndicates deliberately imitating sparrow units. The communist New People's Army sometimes claims responsibility for the deaths, but usually no such claims are made.

Casabar's killing is a case in point. Casabar, a comptroller, was carrying about $500, which was taken after he was slain. The killers could have been robbers, and no one claimed responsibility for his murder.

Ramos said today the military is forming "special action units" to help regular police and constabulary forces try to track down the sparrows. "What is important is to reassure the people of Metro Manila that the government is in control of the situation," Ramos added.

Sparrow units are believed to have been the creation of jailed Communist Party leader Rodolfo Salas. The small teams were formed to carry out urban terrorism and also track down and assassinate suspected informers and government infiltrators within the Communist Party ranks.

When a 60-day cease-fire lapsed Feb. 8, military officials warned that during the truce the communists had infiltrated scores of agents into the cities and were set to launch an intensified urban guerrilla campaign.

Military officials believe that about 700 communist sparrows are operating in metropolitan Manila and that assassin teams are also active in Cebu and other urban areas.

Top Communist Party officials interviewed in the days after the end of the cease-fire confirmed that the party planned to step up its campaign in the cities.

Military officials told reporters on March 4 that captured rebel documents revealed plans for a "Scorpio Force" to engage in assassinations, holdups, "carnappings" and bank robberies to create chaos. The sparrow "hit list," according to the alleged captured documents, included high-ranking civilian officials as well as police officers and other law enforcement authorities.

At the time, Brig. Gen. Alexander Aguirre, commander of the capital region's military force, warned city policemen to institute a "buddy system," advising them to patrol in pairs to help protect themselves.

In the wake of the recent shootings, Aguirre deployed a company-sized unit of 50 soldiers to patrol the Tondo area of Manila and ordered a stepped-up use of checkpoints and roadblocks.

"Certainly we do not want this problem to become more serious," Ramos said today.