A roadside bomb exploded near a military truck Monday night in northeastern Sri Lanka, killing 18 soldiers, security officials said as a senior rebel leader declared the country's cease-fire "null and void."

Tamil Tiger rebels and government soldiers also traded artillery shells and gunfire as the air force struck insurgent positions, killing at least 46 fighters, the army said.

The clashes in the northeastern district of Trincomalee and Jaffna, a port on Sri Lanka's northern tip, were among the fiercest since a 2002 cease-fire and as close to open war as the two sides have come in months of back-and-forth attacks.

CountryWatch: Sri Lanka

Still, the government said it had not violated the cease-fire, insisting it sent ground forces into Tiger territory only to end a rebel blockade of a key water source.

"Our military's operation to open the irrigation gates is purely based on humanitarian grounds. It is not an act of war," chief government spokesman Keheliya Rambukwella said.

But a leader of the Liberation Tigers of Tamil Eelam, the group's formal name, said the government's offensive amounted to a "declaration of war."

"Therefore, for us the cease-fire is null and void," said Puratchi, a rebel commander in Trincomalee who only uses one name. Puratchi said he was speaking for the Tigers, but his comments could not immediately be confirmed by other rebel leaders.

The blast that killed 18 soldiers occurred in Kantale in Trincomalee district, an official from the Media Center for National Security said. One soldier was wounded, he said.

The Tigers took up arms in 1983 to fight for a homeland for Sri Lanka's 3.2 million Tamils, who have faced decades of discrimination. Civil war on this tropical island of 19 million people — nearly three-quarters of them Sinhalese — left more than 65,000 people dead before the 2002 cease-fire.

But peace talks have faltered, and rising violence has killed about 800 people, half of them civilians, since April, according to Nordic cease-fire monitoring mission.

The mission's chief, Ulf Henricsson, a retired Swedish general, said the truce was intact only on paper.

"In reality, there is no cease-fire agreement in this area in Trincomalee today, but the paper is still valid," Henricsson said.

"At the moment, none of the parties are interested in talks ... a full-scale war will be a disaster for both sides."

The root of Monday's fighting dates to July 20, when the Tigers shut a reservoir gate in an area under their control east of Trincomalee, 135 miles northeast of Colombo.

The move cut water to 60,000 residents of villages in nearby government-controlled territory. The Tigers said they took action because the government reneged on a promise to build a water tower for adjacent rebel-held areas.

Sri Lanka's military retaliated with four days of airstrikes on rebel bases in the area and deployed ground forces Sunday to take back control of the reservoir.

As the soldiers advanced Monday, they met stiff resistance from Tiger fighters. The military renewed its airstrikes amid artillery and gun battles, spokesman Brig. Prasad Samarasinghe said.

Thirty-five rebel fighters and seven Sri Lankan soldiers were killed in the fighting, Samarasinghe said. The pro-rebel TamilNet Web site said only three Tigers were slain and put the military's death toll at 12, including two officers.

Soldiers also fought the Tigers near Jaffna, in the Tamils' traditional homeland, after rebels fired mortars at troops. Four rebel fighters were killed, Samarasinghe said.

TamilNet did not mention rebel casualties in the Jaffna fighting.

"This is a new development, as earlier fighting was more in the nature of hit-and-run operations," said Jehan Perera, an analyst at the independent National Peace Council. "The country is entering a deeper state of conflict."

Separately, a spokesman for the Nordic observers, Thorfinnur Omarsson, said the monitors had received no formal letter from the rebels saying they were pulling out of the cease-fire.