The rebel Liberation Tigers of Tamil Eelam (LTTE) said Israeli-built jets carried out two sorties over the town of Kilinochchi and the blasts smashed windows of the main hospital.
"Four people were killed on the spot and another died later," LTTE spokesman Rasiah Ilanthiriyan told AFP by telephone from Kilinochchi, 330 kilometres (206 miles) north of the capital.
"We see this as state terrorism and urge the international community to condemn it."
The defence ministry said the airforce had carried out two separate air attacks inside rebel-held territory, but could not say if there was an attack just outside the Kilinochchi hospital.
"We have bombed a Sea Tiger base at Mannar (south-west of Kilinochchi) and a location about 10 kilometres south-east of Kilinochchi," defence ministry spokesman Prasad Samarasinghe said.
He said there were no details of casualties.
Asked if the airforce had bombed a location near the Kilinochchi hospital, Samarasinghe said he had no information about it.
"We have no presence there, so we don't know about it. But, what the airforce told us is that they have bombed areas away from Kilinochchi," Samarasinghe said.
The LTTE said it had lodged a formal complaint with the Nordic Sri Lanka Monitoring Mission (SLMM), which has an office about two kilometres away from the Kilinochchi hospital.
The LTTE said Thursday's bombing was the second since their two-day peace talks with the government failed in Geneva.
Despite the failure of the talks, both sides pledged to respect a February 2002 truce.

Copyright AFP (Agence France-Presse), 2006