The attacks come amid some of the worst violence since a Norwegian-brokered ceasefire came into effect in 2002, raising fears that the island-nation may return to civil war.
"It was a claymore attached to a three-wheeler (auto-rickshaw)," an army official said from the northern town of Vavuniya, just south of rebel territory, where the attack occurred on Monday.
The attack left four soldiers dead and wounded at least 12.
A claymore is a type of anti-personel mine designed to fire steel balls across a wide arc.
Elsewhere a second claymore ambush on an air force convoy near the eastern town of Batticaloa wounded three personnel, one critically, the army said, while a grenade exploded in the northeastern port town of Trincomalee but hurt no-one.
Meanwhile the army also reported that another man, believed to be a Tiger rebel, was killed in the northern town of Jaffna when a claymore mine he was intending to use in another ambush detonated prematurely.
Two civilians were injured in the blast, he said.
The latest deaths bring to at least 64 the number of people killed in the past week, and comes as an envoy from Norway planned to fly in on a last-minute effort to save failing peace talks.
"We ... wish to inform you with sadness that until the hurdles in front of us to attend Geneva talks are removed and a more conducive environment created, our Geneva team is unable to come to the Geneva talks"
S P Thamilselvan,
Tamil Tigers' political wing leader
On Sunday rebels leaders said they would not attend talks with the Sri Lankan government in Switzerland unless they can hold a crucial internal meeting first.
A pro-Tamil website said that the rebels had formally told Norwegian peace negotiaters they would not attend.
"We ... wish to inform you with sadness that until the hurdles in front of us to attend Geneva talks are removed and a more conducive environment created, our Geneva team is unable to come to the Geneva talks," S P Thamilselvan, the rebels' political wing leader, said in a letter to the Norwegian Embassy in Colombo.
The talks have been scheduled for April 24-25 to discuss how to properly implement the 2002 ceasefire.
Rebel commanders cancelled a trip to the internal meeting on Saturday because of the presence of Sri Lankan naval ships.
The government said the four ships had been deployed to ensure the rebels' safety and that they had told the guerillas about its presence in advance.
The transport squabble is the latest dispute to strain relations between the government and the Liberation Tigers of Tamil Eelam, who began fighting in 1983 for a separate state for minority Tamils, claiming discrimination against them by the country's Sinhalese majority.
Sri Lanka's government agreed to provide helicopters for top rebel officials travelling through government-held territory as part of the 2002 truce, but has recently turned down several requests because of increasing violence.
The development comes amid some of the worst violence in Sri Lanka since the cease-fire.
More than 45 people have been killed in the north and east since last Monday, in attacks blamed on rebels and fighting between ethnic Tamils and the island's majority Sinhalese.