COLOMBO, Sri Lanka (AP) _ Tamil rebels launched heavy attacks on northern military installations today and the government rushed in reinforcements for what could be a decisive battle in Sri Lanka's civil war.

More than 1,400 combatants have been killed since the separatist conflict in the country's north and east exploded into full-scale war again 10 days ago, the government said. No figures were given for civilian casualties, also believed to be high.

Additional soldiers were sent by air and sea to the northern city of Jaffna to bolster defenses at two naval bases, a military airfield and an army garrison, said Deputy Defense Minister Ranjan Wijeratne. He refused to give figures.

Tamil Tiger rebels today attacked the bases, airfield and garrison with rocket-propelled grenades, automatic weapons and suicide squads of guerrillas carrying explosives, Tamil sources said on condition of anonymity.

The sources said the militants were mustering men and weapons for an all- out onslaught on Jaffna Fort, where the largest military garrison in the region is located.

Jaffna, a rebel stronghold, is on the northern tip of this island nation and 200 miles north of the capital Colombo.

The government claimed to have recaptured all major towns in the east this week.

''Right now we have liberated the entire eastern province which includes the districts of Ampara, Batticaloa and Trincomalee,'' minister Wijeratne told reporters in Colombo.

He said that despite the fighting, the government is determined to go ahead with political moves to grant greater political and administrative autonomy to the Tamil minority in the north and east.

''We regard the fighting as a minor aberration. The political process will go on,'' Wijeratne said.

The government appealed to foreign governments for urgent medical assistance.

Government forces, under orders to crush the rebels ''wherever they exist,'' were using helicopter gunships and bombing runs by ground support planes to defend the northern bases, military officials said.

The 200-year-old Dutch fort in Jaffna has been under severe pressure from the dominant Tamil Tiger rebels since fighting broke out between the rebels and government troops on June 11.

The Defense Ministry says several attacks on the fort have been repulsed. About 150 troops and police defend the 55-acre stronghold.

The latest round of fighting broke out June 11 after a Tamil resident in the eastern town of Batticaloa complained he had been manhandled by Sinhalese police. Tamil tigers then overran a police station and combat escalated from there.

Sri Lanka's Tamils make up 18 percent of the country's 16 million people. The Sinhalese make up 75 percent of the population.

Military officials, including Defense Secretary Gen. Cyril Ranatunge, said that since June 11, 800 rebels, 278 policemen and 219 troops have been killed. They did not say where or when the deaths occurred. In addition, at least 500 policemen who surrendered to the rebels are missing.

Officials say civilian casualty figures are unknown but also high since the military relied on air attacks to neutralize rebel positions in populated cities in the north and east.

Rebel statements issued by their London office claim that more than 100,000 Tamils and Moslems have fled their homes in the region this month.

The Tamil rebels are fighting for autonomy in the northern and eastern portion of the Indian Ocean island nation. The mainly Hindu Tamils say they are discriminated against by the Buddhist Sinhalese majority.

The rebels had taken control of most major towns of the region after Indian troops trying to enforce a peace agreement withdrew in March. The pullout, made at the request of President Ranasinghe Premadasa, followed the Indians' unsuccessful 32-month attempt to disarm the Tamil Tigers.

At least 11,000 people have been killed since Tamil militants began fighting for an independent nation in 1983. In 1987, India brokered a peace accord between the two sides and sent a force to disarm the rebels. Fighting ensued and the Indian troops were withdrawn in March of this year.