The military said one soldier died in a claymore fragmentation mine ambush on the northern Jaffna peninsula on Wednesday.
Later in the day, they said another mine ripped through a police pick-up truck, killing two policemen and wounding two more.
The government has been working on constitutional reform to give more power to minority Tamils, but the Liberation Tigers of Tamil Eelam (LTTE), who wanted a separate Tamil homeland, refused to return to talks.
One Western diplomat said: "The LTTE have a habit of attacking when you least expect it. I still think both sides are going for an escalation."
Some fear that Tamil Tiger rebels may simply be waiting for an opportunity before another attack, perhaps in the capital or maybe in the east, where the Tigers and a group of rival ex-rebels, which the LTTE say is army-backed, are both said to be building up.
Opinions are divided on whether war has already resumed.
Truce monitors describe recent ambushes, attacks, killings, naval battles and air strikes as "low-intensity war", but say the ceasefire still holds in most places most of the time.
Worried by reports of killings by members of the armed forces, India and other coutnries are putting the government under increasing pressure to halt the conflict. Some tourists are staying away and foreign investors are getting nervous.
Iqbal Athas, an analyst for Janes' Defence Weekly, said: "It is almost as if [the government] has become more restrained.
"But the LTTE have clearly not given up violence. There is a lot of diplomatic work going on behind the scenes."