Sri Lanka's separatist Tamil Tiger rebels said Monday they are ready for a cease-fire with the government but told the United Nations in a letter that they refuse to lay down their weapons. The Tamil Tigers' political chief, Balasingham Nadesan, said in the letter that international calls for the rebels to lay down their arms are "not helpful for resolving the conflict" and that the weapons "are the protective shield of the Tamil people and their tool for political liberation." "We are ready to discuss, cooperate, and work together in all their efforts to bring an immediate cease-fire and work towards a political settlement," Nadesan wrote in the letter to the UN, which also was sent to Britain, Japan, Norway and the United States. Prime Minister Ratnasiri Wickremanayake has earlier rejected calls for a cease-fire, saying his government will only accept an unconditional surrender by the Tamil Tigers. In recent months, the rebels have suffered unprecedented military defeats, losing most of their strongholds to government forces. They are boxed into a tiny sliver of land in the northeast along with tens of thousands of civilians. The government says it will soon take the remaining rebel territory. In his letter, Nadesan also urged the countries to reconsider their opposition to an independent state for Sri Lanka's ethnic minority Tamils, for which the rebels have fought more than 25 years. Ethnic majority Sinhalese have long dominated the nation's government. The international community "must re-examine our point that an independent state is the only permanent solution to the Tamil-Sinhala conflict," he wrote. Last Friday the rebels used two light aircraft in a daring suicide attack targeting the country's air force headquarters and an air base. The government said it shot down the planes before they could reach their targets. More than 70,000 people have been killed in the civil war.