ONE OF the world's longest-running civil wars appeared to be in its final, bloody phase last night after Sri Lankan troops took control of the island's entire coastland, completely encircling rebel fighters in a space just 1.2 square miles, and cutting off a possible escape by sea for their senior leaders. The country's president predicted that the fight would be over in a matter of hours.

While the move will surely mark the military defeat of the Liberation Tigers of Tamil Eelam (LTTE), the fate of anywhere up to 80,000 civilians trapped in a tiny patch of land with the rebels was last night unclear. International officials said they were extremely worried about what might play out in the next few hours. "We are gravely concerned for the safety of between 30,000-80,000 civilians still in the war zone," said Gordon Weiss, a UN spokesman in Colombo. The UN has estimated that 7,000 civilians have already been killed and a further 16,700 wounded since the beginning of the year.

Even with the conflict seemingly in its final stages and with the Sri Lankan authorities having repeatedly dismissed international calls for a ceasefire to allow the evacuation of civilians, officials were last night still working for a negotiated end to the fighting. The UN secretary-general, Ban Ki-moon, sent his chief of staff, Vijay Nambiar, to Sri Lanka for a second time to try to bring the conflict to a peaceful conclusion.

In reality, it would be remarkable whether Nambiar, or indeed his boss, could do anything to stop the Sri Lankan military machine. After a brutal civil war in which the LTTE has repeatedly used suicide bomb attacks against civilians and military targets, President Mahinda Rajapaksa has long made clear his intention to crush the remaining 1,000 rebel fighters. Even threatened by the US with the blocking of a $1.9bn loan from the IMF, Rajapaksa on Thursday evening predicted that the operation would be completed within 48 hours. As he prepared to return home yesterday from a trip to Jordan, he announced: "I will return to Sri Lanka as a leader of a nation that vanquished terrorism."

The last remaining fighters are surrounded by around 50,000 government troops. In recent days, thousands more civilians have fled the intense shelling of the conflict zone by government forces. The UN said around 20,000 had left in the last couple of days and as many as 4,500 may have escaped yesterday alone. Anywhere up to 200,000 civilians have been able to escape and are currently being interned in refugee camps surrounded by razor wire.

In addition to the uncertainty hanging over the future of those civilians still trapped – reportedly being prevented from leaving by the LTTE fighters – the fate of the rebels' reclusive leader, Velupillai Prabhakaran, and his top deputies, is also unclear. Troops have been scouring the war zone for Prabhakaran, and officials say they believe he is still there, but there has also been speculation that he has already escaped. It has long been said that the rebels' leader wears a small canister of cyanide around his neck and has ordered his bodyguards to shoot him dead rather than allow him to be captured alive.

Even at this late hour, the rebels have been repeating their calls for the government to enact a ceasefire, and restart talks that broke off last year. Selvarasa Pathmanathan, who heads the rebels' international relations department, told journalists that the group welcomed US President Barack Obama's call last week for a peaceful end to the conflict and said they would do "anything that is necessary" to spare civilians. He did not say whether the rebels were prepared to lay down their arms.

Precisely what the conditions are for those still in the war zone is unclear. Even before the rebels were completely surrounded, the thousands of civilians were packed together under tarpaulin shelters, dug into the sand. Food, water and medicine were in short supply and sanitation has been abysmal. But the past week has apparently seen an escalation in shelling, despite the government's undertaking not to use heavy weaponry. Health officials claimed that up to 1,000 civilians were killed as government shells continued to fall, including strikes on the last remaining clinic, set up in a school.

The LTTE has for been fighting for an autonomous homeland for Tamils, which is says are discriminated against by Sri Lanka's Sinhala Buddhist majority.

Rajapaksa, whose brother heads the powerful defence ministry, has said he is open to a political settlement that would help appease the Tamil community, but only after the rebels – who once controlled a large part of north and eastern Sri Lanka – have been militarily ­defeated.

But analysts say that even if, as seems all but certain, the last remaining rebel fighters are killed in this current operation, sufficient numbers of LTTE have probably already escaped to allow an on-going guerilla operation using hit-and-run tactics.