By Simon Gardner

COLOMBO, Oct 23 (Reuters) - Sri Lanka's government said on Tuesday the door was still open for Tamil Tigers to talk peace, a day after the rebels mounted their biggest-ever suicide operation with a ground attack backed by air strikes.

Foreign Secretary Palitha Kohona said the attack, in which 13 servicemen and around 20 Black Tiger suicide fighters were killed, had not changed the government's desire for a negotiated end to a two-decade civil war in which 70,000 people have died.

However the government has declared it aims to destroy the Liberation Tigers of Tamil Eelam militarily and evict them from all territory they control in the far north, while the separatist Tigers demand an independent state -- leaving the sides deadlocked.

"Yesterday's attack was just another pin-prick. It is not going to change anything on the ground. It has very little military significance," Kohona said of Monday's pre-dawn attack, in which Black Tigers infiltrated a base in the northern district of Anuradhapura and rebel light aircraft dropped bombs.

The Tigers claimed to have destroyed eight aircraft parked at the base during the assault, including helicopter gunships and aerial reconnaissance craft, in what analysts said was a body-blow to the military. The government said three aircraft were damaged.

"The situation remains that the government is committed to bringing this conflict to an end through a negotiation process rather than through a military process," Kohona added.

"However it is important that both sides come to the table. The government is ready and willing to be at the table, but the LTTE so far has indicated no intention of coming to the table."

The Defence Ministry on Tuesday posted pictures of the slain suicide fighters sprawled on tarmac at the base, some charred, one with eyes wide-open and one with a gaping hole in his head.

The dead were then stripped, their naked bodies piled into the back of a tractor trailer, and driven along the road in full view of the public.

AIR ATTACKS

The Tigers first used their fledgling air wing of light training aircraft modified to carry bombs earlier this year with raids on an air base adjacent to the island's only international airport and oil installations north of the capital, and have warned more such attacks could follow.

Monday's attack in the north, where renewed civil war is now concentrated after troops captured swathes of Tiger territory in the east of the island, comes after a string of clashes that have killed around 5,000 people since early 2006.

While the government has had the upper hand in recent months, analysts say there is no clear winner on the horizon and fear the conflict -- which has been marked by a rash of extrajudicial killings and alleged human right abuses -- could grind on for years.

The United States called overnight on Sri Lanka to reconsider its opposition to calls by the United Nations and rights groups for an international rights monitoring mission.

"We remain concerned about the deteriorating humanitarian situation in Sri Lanka," U.S. State Department spokesman Sean McCormack said in a statement on the department's Web site.

"An international human rights presence in Sri Lanka would be an important step in improving human rights, accountability and the rule of law and ultimately resolving the conflict in Sri Lanka."

The government has repeatedly rejected calls for an international rights monitoring mission to the island. Kohona said Sri Lanka simply needs help building up its own capacities.

"We are little confused as to the basis on which an assessment was made that there was a deteriorating humanitarian situation," Kohona said.

"I think it will be an unnecessarily complicating factor to introduce a foreign element with judgementative (sic) capacity into our environment."

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