Sri Lanka's Tamil Tiger rebels have announced they are suspending peace talks with the Colombo Government due to their "displeasure" at the handling of some "critical issues".
The country has been wracked by two decades of civil war
The Liberation Tigers of Tamil Eelam (LTTE) said it was committed to a settlement to the two-decade conflict, but that progress had to be made on the ground before the settlement proceeded.
In what correspondents have described as the most serious setback since the peace process started more than a year ago, the rebels said they would not attend a meeting hosted by Japan in June to rally international support for the island's peace efforts.
The government has responded by saying concerns about the process should have been addressed within peace procedures.
There was no hint of a return to war in the Tigers' letter, and observers believe there will be feverish diplomatic activity over the next few days to try to work out a compromise.
The Tamil Tigers announced their intentions in a four-page letter to Prime Minister Ranil Wickramasinghe from their chief negotiator, Anton Balasingham.
"In accordance with the decision of our leadership, I am advised to bring to your urgent attention the deep displeasure and dismay of our organisation on some critical issues relating to the ongoing peace process," said the LTTE's chief negotiator, Anton Balasingham.
Among the Tigers' grievances are:
- Their exclusion from reconstruction talks in Washington on 14 April, and a more general perception that they are not receiving the full economic rewards of peace
- The failure, as the Tigers see it, of dividends in peace talks to transfer to security withdrawals on the ground
- The disparity between the relative calm of the government-held north-east and continuing violence in Tiger-held areas.
Correspondents stress that there was no hint of a return to war in the Tigers' letter.
The decision to halt talks will also affect the seventh round of peace talks due to open in Thailand on 29 April.
In response, the government said: "If there is any complaint about the peace process they should have referred the matter to the Norwegians who are the facilitators of the peace process.
"We are committed to continue with the peace process while addressing the shortcomings of implementing the ceasefire agreement," prime ministerial spokesman Gayrika Perusinghe told the BBC.
Previous rounds of negotiations have succeeded in consolidating the February 2002 ceasefire, setting up provisional administrative arrangements and addressing a final political settlement.
The Tigers dropped their demand for independence and said they would settle for regional autonomy - a major concession.
The government also gave ground - this was the first time it had agreed to share power with the Tamil Tigers.
But the return of tens of thousands of refugees to the government-held north-east of the country, and the disarmament of the Tigers, have both been sticking points.
Tamil Tigers feel they are not seeing the full rewards of peace
Five people were killed and hundreds fled their homes in riots between Tamils and Muslims in north-eastern Sri Lanka last week.
The riots were sparked by an alleged Tamil Tiger kidnapping - an allegation the group denies.
Until a ceasefire first declared in December 2001, the Tigers had been fighting since 1983 for a separate state for Tamils in Sri Lanka's north and east.
They argued that the Tamils were discriminated against by the majority Sinhalese population.