Serious crime ranging from robberies to assaults of bus drivers to outright slayings has been climbing on the Metrobus system, prompting community members, union officials and bus operators to call for increased security.

But the process of fighting crime on the agency's some 1,500 buses is not easy or immediate, according to a new Metro report slated to be presented to board members on Thursday.

Metrobus crime accounts for just under 7 percent of all serious crimes, known as Part 1 crimes, recorded by the Metro Transit Police. However, such crimes rose in 2012, with an average of 11 serious crimes per month on Metrobuses compared with seven in 2011, according to the report.

The mounting problems include three homicides that occurred on buses or at bus stops last year, far more than the single slaying Metro typically has on the entire system in a single year. The dead include a woman who was fatally shot as she held her toddler in her arms.

Bus drivers reported being assaulted more frequently, as well, with 74 cases from January through mid-December, compared with 65 for the same period in 2011.

The agency also threatened this fall to cut night bus service on some routes in Southeast D.C. because teens were throwing rocks at buses there.

"We have seen a slight increase in Metrobus-related crime, and we have redeployed resources to address it," said Metro spokesman Philip Stewart. "Even though crime is low overall, we have taken and will continue to take action to respond to trends."

Metro does not have enough police to place officers on all of its buses, let alone every line. It also takes time to hire more of them.

The transit agency created 32 new police officer positions in July dedicated to Metrobus security, which would double the number of officers in that unit. But the agency is still hiring and training the officers. It doesn't expect them to be available to patrol buses until late 2013.

In the meantime, it has been conducting at least 10 targeted patrols per month, in which police in uniform and plain clothes saturate a particular route or line. D.C. police are participating in the patrols when they are available, Stewart said.

But some efforts to promote safety have been blocked. D.C. officials slammed the agency's plan to cut bus service due the rock-throwing. Instead, the agency is distributing fliers urging teens not to throw rocks at buses.

Metro also has installed protective shields inside 52 buses to protect drivers, but that was a scaled-down version of its original plan to add them to 301 buses -- a proposal that was changed because board members in December 2011 questioned the plan for sending the wrong message to riders.