A call for negotiations last Friday from the leader of the Liberation Tigers of Tamil Eelam drew a wary but welcoming response from Sri Lanka's main opposition party. ''This is a major move by the Tigers, and it is a very positive one to which the government must respond,'' said the leader of the United National Party. This response is promising because for too long the opposition and the governing People's Alliance of President Chandrika Kumaratunga have competed to appear the more inflexible foe of dialogue with the Tamils.

Because Washington maintains warm relations with the Sri Lankan government, even providing training and arms sales to its armed forces, and since the Tiger leader Velupillai Prabhakaran called for third-party mediation in his offer of negotiations, the United States could play a crucial role in ending Sri Lanka's long nightmare.

The State Department has been reluctant to become involved in the conflict because neither side had been willing to accept the premise of a negotiated solution, as the antagonistic parties did for the Oslo accords in the Middle East and the peace talks that George Mitchell guided in Northern Ireland. Even now the State Department does not want to rush ahead of events.