Defence Ministry spokesman Brigadier Ruwan Wanigasooriya said Sri Lanka has banned 16 organisations, including the LTTE, in accordance with the UN Security Council resolution 1373.
These organisations have been classified as "terrorist" under the new government proclamation, he said.
From now on it will be illegal to have any contact with these organisations, Wanigasooriya said.
He said among the organisations proscribed are the LTTE's Trans National Government of Tamil Eelam (TGTE), Global Tamil Forum (GTF), which is active in Europe, and the UK-based British Tamil Forum (BTF).
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The announcement came as the UN Human Rights Council in Geneva last week adopted a resolution calling for an international investigation into alleged war crimes committed by Sri Lankan troops during the final battle with the LTTE that ended in 2009.
Sri Lanka had accused the LTTE front organisations of moving the resolution against the country as means to take revenge from Colombo for militarily crushing the LTTE, which had fought for nearly 30 years for a separate Tamil homeland under Velupillai Prabhakaran.
Up to 40,000 Tamil civilians were killed at the end of the separatist war, rights groups and UN experts have said.
The government will urge for an investigation into these organisations and their funding of terrorism, he said.
Sri Lanka will also request the governments to classify LTTE front members as offenders and name them as wanted by the Interpol for terrorist activity.
Sri Lankan nationals and political parties will be forbidden from making contact with such organisations.
The individuals belonging to these organisations would face arrest under anti-terrorism laws when travelling to Sri Lanka, Wanigasooriya said.