Suspected guerrillas shot and wounded a police officer on Thursday in the northeastern town of Trincomalee in the latest of 10 bombing and shooting attacks since Tuesday, military spokesman Brigadier Daya Ratnayake said.
In response, police and military reinforcements were sent to the country's eastern region to secure power lines and a key route used by thousands of troops and civilians, he added.
The heightened alert came a day after separatists shut down political offices in government-controlled areas, accusing the government of failing to protect them while travelling through those areas.
Dozens of Tamil officials started pulling back on Wednesday into guerrilla-held territory from the eastern cities of Trincomalee, Batticaloa and Ampara and said they were also considering pulling out of the north.
Confirming the withdrawal, Liberation Tigers of Tamil Eelam (LTTE) spokesman Daya Master told journalists that he was "not satisfied with the security measures of the government, which has compromised the safety of our members".
LTTE political chief SP Thamilselvan had demanded on 30 June that the government boost security for separatists travelling through government-held areas within two weeks.
He said the government risked the collapse of the February 2002 ceasefire that ended the country's two-decade long brutal civil war.
The deadline expires on Thursday.
The government on Monday agreed conditionally to increase the Tigers' security escorts, but the guerrillas were not satisfied.
They have demanded better protection after a bomb narrowly missed a bus carrying 41 officials last month. Subsequently, two senior LTTE members were gunned down. The Tigers blame the military.
"We are very concerned about the eastern situation," Cabinet spokesman Nimal Siripala de Silva said, adding that the government was "worried" about the stability of the truce, but wanted it to be respected and the peace process to continue.
The Tamil Tigers began fighting in 1983 for a separate homeland for minority ethnic Tamils in the country's north and east, citing discrimination by the majority Sinhalese.
The conflict killed nearly 65,000 people before the February 2002 ceasefire. Peace talks since then have been stalled since 2003 over rebel demands for wide autonomy.