Norwegian special envoy Jon Hanssen-Bauer has arrived in Sri Lanka, just a day after a suicide bombing killed almost 100 sailors.
The buses carried navy servicemen on leave, the military said
Mr Hanssen-Bauer will discuss details of forthcoming talks between government and Tamil rebels, but observers say the fresh violence could derail them.
The rebels did not confirm or deny the attack, in which an explosives-loaded truck rammed a military bus convoy.
Ninety-seven people were killed and over 100 others wounded.
The attack took place near the town of Habarana, 190km (120 miles) north-east of the capital Colombo, on Monday.
The servicemen were said to be on leave at the time.
Cremations of the dead are due to begin on Tuesday, an army spokesman said.
Brig Prasad Samarasinghe told the BBC News website that 116 people were wounded in what was the worst such blast in the conflict.
DESCENT INTO VIOLENCE
16 October 2006: 99 killed in suicide attack on navy convoy
11 October 2006: 129 soldiers die in fighting between army and Tamil Tigers
4 September 2006: Troops take control of a crucial area near Trincomalee
14 August 2006: 61 schoolchildren killed by air force bombs, rebels say
7 August 2006: 17 local employees of a French charity shot dead in the town of Muttur
26 June 2006: A suspected suicide bomber kills a top Sri Lankan general near Colombo
15 June 2006: At least 64 - many children - die in mine attack on bus
He denied that the Sri Lankan government had launched air strikes on Tamil Tiger positions after the attack, saying the strikes had been launched before the incident to repulse rebel artillery attacks.
Rasaiah Ilanthirayan, a spokesman for the Tamil Tigers, told the pro-rebel website TamilNet that the incident was not a "suicide mission".
The incident comes after at least 129 Sri Lankan soldiers were killed and 300 injured in fighting on Wednesday - the worst single day of casualties for the military since a ceasefire was signed in 2002.
A spokesman for President Mahinda Rajapakse called the attack "cowardly", pointing out that it took place "in a civilian area away from the area of armed confrontation".
During the air strikes against Tiger positions in the north on Monday, an air force fighter jet crashed in a lagoon about 30km (18 miles) from Colombo, although the pilot ejected to safety, the military said.
UN Secretary General Kofi Annan was "alarmed" by violence in Sri Lanka in the past few months, including Monday's bombing, his spokesman Stephane Dujarric said.
The rebels did not confirm or deny the attack, but said it was justified.
Rasaiah Ilanthirayan, a spokesman for the Tamil Tigers, said: "To this moment, I do not know for sure who did this."
But he indicated that it could have been the Tigers.
"There is a possibility of that kind of targeting, but this particular target, I am not sure until I get information from the Eastern Commander," he told the BBC.
A Tamil Tiger spokesman told Reuters news agency that Sri Lankan planes had bombed a village near the north-eastern town of Mullaitivu late on Monday and several civilians were feared killed.
The convoy attack came shortly after Japan's peace envoy to Sri Lanka, Yasushi Akashi, met President Rajapakse in Colombo.
Later in the week, he is due to meet senior Tamil Tigers in the north of the island and envoys from Norway and the US are scheduled to arrive.
The BBC's Dumeetha Luthra in Colombo says that given the climate of violence, there is little optimism about what is likely to be achieved in the Geneva talks.
At least 2,000 people have been killed in violence this year in Sri Lanka, the military and ceasefire monitors say.
Before the 2002 ceasefire, more than 60,000 people were killed in two decades of civil war.
The Tamil Tigers are fighting for an independent homeland in the north and east of the country, and claim that ethnic Tamils have suffered decades of discrimination at the hands of Sri Lanka's Sinhalese majority.