Under pressure from the United States, the 25 EU members on Thursday agreed, from next week and at the latest "before June", to place the LTTE on its list of terrorist organisations.
The move will notably allow greater scrutiny of funds transferred to the rebel group.
"This does not affect Norway. We do not use the EU list of terrorist groups, we use the UN list," Solheim said.
"It certainly means that Norway may become even more isolated in the Sri Lankan peace process, since we will be one of the few countries able to deal completely impartially with both Sri Lankan authorities and the Tamil Tigers," Solheim told Norwegian national radio NRK.
Since April, Sri Lanka has seen an upsurge in violence which has claimed more than 200 lives, undermining the 2002 peace agreement brokered by Norway.
The conflict has caused the deaths of more than 60,000 since 1972.