"The four-month ceasefire which we extended has come to an end," the rebel leader Prachanda, or the Fierce One, said in a statement.

"We want to make it clear our future actions will be targeted against the autocratic regime. We are compelled to go offensive not only for the sake of peace and democracy but for the sake of self-defence."

King Gyanendra, who seized power 11 months ago, and the army declined to match the truce called initially for three months in early September and extended by a month in December. It ended at midnight.

The Royal Nepalese Army was ambivalent about the announcement.

"The army's position has not changed. Whether there is a ceasefire or not, we are acting on the government's mandate. The army will continue to do its duty," a senior army official said on condition of anonymity.

The United States embassy in Nepal said the end of the ceasefire was an unwelcome development.

"We think that it is very unhelpful and we are deeply concerned," a spokeswoman said. The United States, India and Britain cut lethal military supplies to Nepal after the royal takeover.

More than 12,000 people have died in the so-called "people's war" waged by the Maoists since 1996 to establish a communist republic.

The end of the ceasefire will result in increased unrest and violence in the months ahead, said political scientist and retired politics professor Lok Raj Baral.

"It will be back to square one," Baral said." Strikes, conflicts and disturbances will start again. The government will face more confrontations, with the opposition alliance and the rebels."

In November the Maoists formed a loose alliance with seven political parties, saying they would jointly press for democracy.

The parties earlier Monday had called for an extension of the ceasefire as did UN Secretary General Kofi Annan and the European Union last week.

The rebel leader said he "respected and understood" the calls to continue the ceasefire but an extension was not an option.

"The Royal Nepalese Army has compelled us to end the ceasefire. It was not only impossible, but also suicidal for us to extend it further," Prachanda said.

The end of the ceasefire will not signal an end to the alliance with the parties, according to the acting president of the Nepali Congress (Democratic) party.

"The end of the ceasefire will not have any effect in the understanding between the seven-party alliance and the Maoists, though we still continue to reiterate our earlier commitment to peaceful and non-violent agitation," Gopal Man Shresthra told AFP.

The parties and the Maoists have said they will boycott municipal elections called by the king on February 8.

Under the king's "roadmap to democracy," the municipal elections in February will be followed by national elections sometime before April 2007.

Opposition politicians have branded the roadmap a whitewash, and continue to call for a return to multi-party democracy.

The rebels and the opposition parties want to hold constituent assembly elections that would frame a new constitution and define the future role of the monarchy.

In 1990 Nepal established a constitutional monarchy that led to parliamentary rule for more than a decade before Gyanendra seized power.