The Defence Ministry said 20 soldiers and police and one civilian were injured when suspected Tigers triggered a mine against their truck in Batticaloa, 220km east of capital, Colombo, on Thursday.
The military media unit said three police died in the attack, as did one civilian.
Separately, three sailors and a police officer were injured in a suspected rebel mine attack in the eastern town of Trincomalee, 230km northeast of Colombo.
Both Trincomalee and Batticaloa fall within the rebels' envisaged homeland for Sri Lanka's 3.2 million ethnic Tamil minority.
The attacks came as the Sri Lankan parliament on Thursday extended a state of emergency as continuing separatist violence threatened to plunge the island back into civil war.
Trincomalee, which serves as a major base for the Sri Lankan navy, has been hit hard by violence blamed on the separatist Liberation Tigers of Tamil Eelam, who run a de facto state in parts of the north and east of the island.
They routinely deny any role in such attacks.
Batticaloa has also been a hotbed of violence, most of it blamed on the Tigers.